endSessionDialog: Use a 48px icon as per mockups

https://bugzilla.gnome.org/show_bug.cgi?id=722898
endSessionDialog: Simplify CSS padding handling

... so that we add more items to messageLayout without having to
hardcode the same padding in each of the children.

https://bugzilla.gnome.org/show_bug.cgi?id=722898
endSessionDialog: Fix the "Restart & Install" button

This moves the dialog type overriding that gnome-software uses to bring
up restartInstallDialogContent from _sync() to OpenAsync(), in order to
ensure the type is overridden early enough to show the correct buttons.

While at this, make sure the & symbol in the button's label is escaped
to avoid runtime warnings, now that the label actually gets used.

https://bugzilla.gnome.org/show_bug.cgi?id=722898
endSessionDialog: Offer offline updates in the shutdown dialog

This adds a checkbox for installing software updates to the shut down
dialog. The implementation relies on an already prepared offline update
and uses PackageKit's pk-trigger-offline-update helper to trigger the
installation.

https://bugzilla.gnome.org/show_bug.cgi?id=722898
endSessionDialog: Warn when trying to install updates on battery power

Interrupting update installation can mess up the package database quite
a bit and could lead to totally destroying the distro installtion. To
avoid running out of juice during an upgrade, warn when someone tries to
install updates on battery power.

https://bugzilla.gnome.org/show_bug.cgi?id=722898
endSessionDialog: Add extra strings for translation

Add two extra, currently unused strings in order to make the life easier
for translators and avoid breaking the upcoming string freezes.

https://bugzilla.gnome.org/show_bug.cgi?id=722898
window-tracker: Be more cautious when setting focus app

Since we started tracking non-interesting windows, we can no longer
assume that if we manage to find an app associated with the focus window,
it should appear focused - we now can find apps for docks, the DESKTOP
window etc.
To restore the old behavior, make sure that the focus window or one of
its parents is "interesting".

https://bugzilla.gnome.org/show_bug.cgi?id=722928
windowManager: Implement tile previews

Mutter now delegates tile previews to compositor plugins, so
add a simple implementation based on the UI previously provided
by mutter.

https://bugzilla.gnome.org/show_bug.cgi?id=665758
windowManager: Animate tile previews

With tile previews being implemented as Clutter actors in the shell, we
can now easily add fancy animations when showing/hiding the preview.
Besides looking more polished, the animations also help understanding
what will happen to the window when the drag is finished.

https://bugzilla.gnome.org/show_bug.cgi?id=665758
Revert "viewSelector: Give the active page key focus when it is shown"

This broke keyboard navigation in the app picker, which is not an
acceptable regression for the release.

This reverts commit ec2bb039ae7ab3f3504bfe0fdba6c715f5a838fd.
Bump version to 3.11.90

Update NEWS.