Allow Format Icons

Currently the generic video icon is the thumbnail of last resort, which isn't always a good fit for the media in question.  This is a simple patch to allow specifying default format icons to be used instead at this point.

The context for me is jumpy's psuedo-formats, which are pretending to be text files, web pages, etc and could use a better icon when matched by UMS, but I thought it may be generally useful too, i.e flac vs wav etc.