Add possibility to define mimetype rules for menus like nautilus-actions

[Johann-Tree]

I would like to be able to define mimetype rules for a menu like it was possible in nautilus-actions because then I wouldn't need to define the same mimetype rule for every action within that menu.

[bassmanitram]

Hmmm... not a bad idea... Though I think I'll extend it a bit and allow all the filters. Thanks (it may take a while to get done, but I will work on it)