gnome-desktop's background drawing code supports an
XML format for presenting backgrounds based on time of day,
monitor geometry, etc. Now that we don't use gnome-desktop for drawing the
background, we need to implement that support ourselves to maintain
feature parity.
This commit implements that.
https://bugzilla.gnome.org/show_bug.cgi?id=682429
This commit updates the code to use mutter's new background
api, and changes the shell's startup animation to be closer
to the mockups.
Based on initial work by Giovanni Campagna
https://bugzilla.gnome.org/show_bug.cgi?id=682429