Implemented a very basic form of GD-ROM Sense. Only disc swap related Sense currently works. Added some very basic disc swap handling on x86/Windows (press "0">Cancel to remove disc, then "0" again and select the new disc).

Notes:
Disc swapping should work on .gdi files (probably .chd files too).
Swapping with .cdi files is not currently supported because we currently create inaccurate TOC for them.
Only x86/Windows has a disc swap handler right now (ie: you can't swap on Android).