Add Horde_Themes:: library.
Move all themes related code to a single place (includes both image and
sound).
Horde::img(), through Horde_Themes::img(), will now attempt to auto
determine a graphic location. It looks in the following directories with
this priority:
1. App theme
2. Horde theme
3. App default
4. Horde default
This prevents us from having to copy all over all graphics for a theme -
now, only need the graphics that differ from defaults (caveat: if
loading graphics from CSS, copies of graphics need to continue to live
in theme directory).