Refactor the GriloMedia class for multiple plugins

The class Games.GriloMedia currently is only able to use the grl-thegamesdb plugin. Now other Grilo plugins may be used such as the new grl-steam-store plugin which will essentially share the same logic as this class with a few exceptions. For instance, the class assumes metadata is resolved from a title and a mime type and the Steam store plugin will use the app id from the Steam registry.

Do you think that's the way to go or do you have a different idea?