gtkexcepthook only for those that run svn versions; release user should not be...

gtkexcepthook only for those that run svn versions; release user should not be bugged with TBs as they should be minor to overall functionality and he should not care. if he is advanced he knows how to find out what is wrong (how to get to see a traceback etc); also moving get_avatar_pixbuf_from_cache to gtkgui_helpers (all in one commit because of import issues)