Background: ensure we set the vera-color if we should.
src: moved AverageColor.vala to background/
desktop.vala: removed legacy update_background method.
Background: added settings for wallpaper randomization.

Enable with background-random-enabled and tweak the timeout by changing
background-random-timeout.

The wallpapers will be searched in background-search-paths (with the background-include wallpapers and
without the background-exclude ones)