Make all games coming from a file implicitly have a "local" cover
Migrated from https://bugzilla.gnome.org/show_bug.cgi?id=782420
The code generating game objects is quite similar in many cases and should be generalized whenever possible. All games generated from a file that the user control are eligible to have a so called "local" cover (coming from an image file with a name similar to the game's file, so this is a good candidate for generalization.
A way to do so would be to we could make the Game interface to require having a MediaSet (even an empty one), and use it to by default use it and the URIs it contains to look for local covers.