Large KGrTheme class cleanup. Features of the new theme code include:

* Everything that gets rendered from SVG files is now cached in a pixmap cache. This reduces load time considerably for themes.
* Pixmap caches are invalidated automatically when the theme files are updated.
* Pixmap caches are set to have a 3MB limit. This is large enough to store several complete cached copies at different resolutions before the cache runs out of space.

Note that this limit is PER THEME.