[icon-theme] Support and cache fallback icons
[icon-theme] Support and cache fallback icons

Support look-up and caching of icons that have fallback providers (e.g.
gnome-dev-cdrom is a fallback for gnome-dev-cdrom-audio).

clutter-project/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx,clutter-project/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx