Fixing bug: icon images were not being updated during a refresh

* Imlib2 cache system was holding previous icons in memory.
* Now we are clearing the cache during a refresh, and enforcing it’s size allocation.
* Forcing the release of fixed icons from memory during each icon class unload
* Also adding a configuration setting for the the cache size so we can control kdesk’s memory working set.
Merge pull request #71 from KanoComputing/refresh-icon-images

Fixing bug: icon images were not being updated during a refresh