Code cleanup.
Fix a crash when creating box images. (Probably related to #915 and #877)
Make sure cover art files are deleted when a game’s box image changes.
Code clean up, improves game info lookup performance by calculating thumbnail images off the core data (i.e. main) thread.
Keep track of cover art source.
Add ‘Cancel Cover Art Download’ menu item to collection view context menu. (only visible for single games that are scheduled for syncing)
Try if making the managedObjectContext retain objects fixes a weird crash in -save method.
Removed RetroUSB automapping support for now to reduce conflicts

We cannot reasonably tell this apart from other clones that have the same vendor/product ID.