background: Fix cancellable issues

If we have the following sequence:

    cache.getImageContent({ filename: "foo", cancellable: cancellable1 });
    cache.getImageContent({ filename: "foo", cancellable: cancellable2 });
    cancellable1.cancel();

Then the second load will complete with "null" as its content, even though
it was never cancelled, and we'll see a blank image. Meanwhile, since the
second load simply appends to the list of callers for the second load,
cancellable2 does absolutely nothing: cancelling it won't stop the load,
and it will still receive onFinished handling.

To prevent this from happening, give the actual load operation its own
Gio.Cancellable, which is "ref-counted" -- only cancel it when all the other
possible callers cancel.

Additionally, clean up the large nested loops by splitting out duplicated
code and other stuff.

https://bugzilla.gnome.org/show_bug.cgi?id=722149
layout: Add different groups in the LayoutManager for discrete UI components

This helps take cruft out of the uiGroup, and ensures that components remain
stacked properly on top of each other. In the future, we'll use this group
to ensure that grabs are ordered properly, as well.
layout: Add sessionGroup / systemGroup to better-define layers

In order to build a better transition animation from the lock screen, we
need to split the world into layers, as per this reference:

https://raw.github.com/gnome-design-team/gnome-mockups/master/system-lock-login-boot/system-layers2.png

Everything that pertains to the user's session is in the "session group",
which includes the window group, overview, message tray (for now),
keyboard, OSDs, menus, etc.

For implementation sake, we did not match this mockup exactly. The new layers
look like this, from top to bottom:

 * Stage
   * Magnifier (clones the uiGroup)
   * uiGroup
     * overlayGroup
       * menuGroup
     * panelGroup
     * screenShieldGroup
     * sessionGroup
       * top_window_group
       * other boxes (trayBox, keyboardBox, etc.)
       * other groups (osdGroup, switcherPopupGroup, etc.)
       * overviewGroup
       * window_group
     * systemGroup

The "session startup" animation now only zooms in the sessionGroup.

The panel is now outside the session, as it needs to sit above the screen
shield. This also means that it's not zoomed in as part of startup. I think
this is OK.

This also means that the lightboxes that the screen shield uses to fade out
the screen have to go in a new group, above the panel. This is known as the
overlayGroup, which has no relation to the old mutter group of the same name.

We also change the screen shield to put the lockDialogGroup in the system
group, and put the lockScreenGroup in the screenShieldGroup, which means
that the layer stacking is correct. Note that we don't hide the session
group in the lock screen yet, which is something I want to do.

Since not a lot of items need to be in the uiGroup anymore, we've removed
the Main.uiGroup fallback; others should use sessionGroup instead, when
appropriate.
layout: Replace SystemBackground with CSS on the systemGroup

This merges the implementation of the noise texture on the lockScreenDialog
and the startup animation -- they now just use the systemGroup.
screenShield: Don't hide windows until we've fully locked the shield

Revert the commit that ties hasWindows to the session mode, as we
need to do the "transition" manually. Instead, show/hide the
sessionGroup ourselves.

For optimization purposes, we also need to hide the window groups
when in the overview. Ideally, these would be culled out by Clutter,
but they are not from experimentation.
viewSelector: Move to a sync() model for deciding which page to show

https://bugzilla.gnome.org/show_bug.cgi?id=722196
overview: new transition