Refactor thumbnails

This refactors how thumbnails are cached to make their cache directory name more relevant. This also draws the icons at the same size as the covers, and make icon ojects return a pixbuf as we don't need GIcon anymore. This makes hand is a first step in making game-embedded icons and externaly provided covers be handled the same way.

This also fixes some thumbnail sizing issues that were caused by differences in how covers and icons were treated.