Rework the code provided by Kadlcik Libor for loading/showing icons in Openbox menus.

Rework the code provided by Kadlcik Libor for loading/showing icons in Openbox menus.

This changes how the imagecache works, you can load an image into it directly,
or you can load it by name (then it will load it from a file on disk).

NOTE: The name part is incomplete, as it needs to use the freedesktop.org icon
spec to search for the right file.  Also to resize it should look for another
icon on disk with the same name but different size (icon themes).