Add repeating A and B keys

[lesleyrs]

Title says it all, more convenient repeat feature like BGB has.

Also I get 4x speed on Peanut-GB where SameBoy is getting me like 1 fps on postmarketOS (linux phone)

It's able to work on touchscreen without changes by modifying the onscreen keyboard which is quite cool 👍

Edit: I noticed there's a problem with the repeating keys, where if u press another key it will stop repeating even if it's still pressed? Still going to use it though.

[deltabeard]

Thank you for your contribution! Would you be able to only include the first two commits in this pull request (9cdbfdc and 3a065f8) please? Since the following commits, while useful, aren't relevant to the pull request. Thanks!