Modal stack: fix handling of destroyed actors

Destroyed modal actors should be completely removed from the modal
stack automatically, including leaving modality if needed.
This allows for destroying modal dialogs without calling close().

https://bugzilla.gnome.org/show_bug.cgi?id=619955
Panel: fix finding leftmost and rightmost buttons

Previous code would access the array element before checking that
the index was within bounds, and therefore cause a TypeError.
It wasn't noticed earlier because at least one visible children
is in each panel box in all session modes.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
UserMenu: split user avatar into its own widget

This way, it can be reused in the lock screen without code duplication.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
St: add :disabled pseudo class when a button is not reactive

The :reactive property is used on StButton to like the :sensitive
property on GtkWidgets, that is, to indicate that the user is not
(yet) expected to click the button, and therefore should affect
styling too.
This allows to remove some code at the JS layer.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
St: don't attempt to give focus to non reactive actors

Non reactive actors don't expect to be interacted with, and thus
should not get keyboard focus.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
MessageTray: rework icon handling to split model and view

To allow more than one summary icon actor for a source we split
the model of the source icon (which is iconName, if the default
implementation is used, or a GIcon otherwise) and replace
createNotificationIcon() with a generic createIcon(size). Also,
the actual source actor is split into a separate class, that handles
the notification counter automatically.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
ModalDialog: add 'default' pseudo-class to default button

Add 'default' parameter to setButtons, that controls the binding
of Return (unless overridden) and applies the 'default' pseudo-class.
Currently it has no effect, but it will start having after the
login dialog redesign.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
screenShield: add initial functionality

We are replacing the gnome-screensaver module with with a screen shield
that is part of gnome-shell.

This patch fades out the screen on idle and shows a shield with a background
image when there is activity again. The shield can be removed with a key or
button press.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
Split some common code out of gdm/loginDialog

This will be reused by session unlocking.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
Add UnlockDialog for unlocking the screen shield

When the screenshield is deactivated, instead of going back to the
session immediately, prompt the user for authentication.
This essentially reinstates what used to be provided by gnome-screensaver.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
ScreenShield: improve locking/modal policy

Ensure that the lightbox is above everything (including the screenlock
itself) when fading in - this allows for fading while showing the
unlock dialog. Also, don't pushModal again when already locked, or
we won't get out of it.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
ScreenShield: use LayoutManager for creating the actor

This ensures that the screen shield is created at the right
stacking level, so the message tray is visible in the lock screen
(showing PAM messages, critical notifications and the on screen
keyboard)

https://bugzilla.gnome.org/show_bug.cgi?id=619955
ScreenShield: implement curtain design

This separates the screen shield into two main screens. One is
the lock screen, and it is shown when coming back from idle status
and when failing authentication. The other is the actual unlock
dialog.
Moving from the first to the second is possible by pressing Escape
or by dragging an arrow on the bottom on the screen.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
Consolidate creation of login and unlock dialog in the screenshield

The design calls for the curtain to appear in the gdm greeter too.
Implement this by having the screenshield manage the login dialog
(delegating its creation to SessionMode).

https://bugzilla.gnome.org/show_bug.cgi?id=619955
Add a clock to the lock screen

Start implementing the lock screen design by adding a big white
clock in the middle.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
Show the panel above the screenshield when locked

Track locked status and use it to provide a reduced version of
the panel in the locked screen. Accessibility, input sources and
volume menus are preserved, without the link to the control center.
Network, battery and user menu are reduced to pure indicators,
with no menu.
This is similar to the design but not exactly, because designers
in IRC said that network needs more analysis before exposing, and
because the design didn't account for a11y and IM (so the one menu
metaphor is not really appropriate).

https://bugzilla.gnome.org/show_bug.cgi?id=619955
ScreenShield: show notifications in the locked screen

Track screen lock status in the message tray, and filter banner
notifications. The message tray is completely hidden when the screen is
locked, but exceptions can be made for individual transient notifications,
such as shell messages and the on screen keyboard.
Non transient sources are shown in the middle of the lock screen. Resident
notifications (such as those from Rhythmbox) are shown in full, while
persistent ones are displayed as icon and message count.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
Don't track the status of the screensaver on DBus

We are the screensaver now, and internal objects can track the
locking status better themselves. And to do so, add two signals
to ScreenShield.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
Screen Shield: animate manual locking

When the screen shield is activated from the user menu, animate
it instead of showing it abruptly. Also, ensure that the animation
had time to finish before calling UPower to suspend, to avoid
showing it when resuming.

https://bugzilla.gnome.org/show_bug.cgi?id=619955
Login Dialogs: update styling to match mockups

Remove backgrounds, change button styling, reduce font sizes.

https://bugzilla.gnome.org/show_bug.cgi?id=619955