Fix crashes when dragging and dropping files outside of the gamelist

Prior to this PR dragging and dropping games into a docked window other than the game list would cause the emulator to initialize incorrectly and crash due to an access violation from dereferencing a null pointer (thanks to JMC for pointing this out). Since the idea here is to initialize the game/compatible file regardless of its dropped location within the main frame, it makes sense to actually have this handling code within the Frame code.

The game list is locked into the main frame, so this still retains the same behavior. It just won't crash when drag/drop is done on any other docked window in the main frame.