Context Menu in Bookmarks Dialog

This adds a context menu to the Bookmarks dialog. The reason was, that everywhere in XBMC you can delete/remove items via context menu, except in the bookmarks dialog. Got the idea from here http://forum.xbmc.org/showthread.php?tid=83178&pid=626847#pid626847

Had this one ready for quite some time now, but never cleaned it up, but now here it is ;)