schema: changed defaults in preparation for the 7-preview.
desktop.vala: refactored on_settings_changed().

To keep things in order, we are now using a switch.
This also fixes the bug where vera will re-paint the wallpaper if
background-{exclude,include,paths} have been modified.
desktop.vala: reset the eventual random timeout if the wallpaper has been changed.
Merge branch 'master' into debian
debian/control: depend on libgnome-menu-3-dev.
0.24.0-1