Fix issue #155 : Seeking - for graphs that don't support frame time format but contain video media types, simulate frame back/forward seeking by stepping backwards or forwards by smallest video frame time interval in graph and show effective fps in seek window.

git-svn-id: http://graph-studio-next.googlecode.com/svn/trunk@258 c2f33eb2-41b2-391d-4cb4-d9d0608729ba