[Feature Request] Option to show the emulated GB/GBA's button presses on the emulator window

The idea is simple: it would be an option that, if toggled, any time you press, for example, the A button, the emulator window shows an A as a sign that you pressed that button. This has been implemented in several emulators before, and I think mGBA can really benefit from it.

One of the reasons I usually recommend people to turn this option on when available is because you can check problems with the responsiveness of your controls. That is: if you are pressing the A key in the keyboard or the A button in the controller (assigned to the GB/GBA's A button for this example) and the game is not responding accordingly, is it a problem with the game, the connection or the device? I used stuff like this to check if my keyboard or controller is being registered by the game, and to locate much better where the issue is coming from. I think that, for future issues with controllers or the keyboard (at least in the "desktop versions"), it can help to locate faster the lack of response (either total or intermitent) source. Also, for the planned TAStudio-like features suggested in https://github.com/mgba-emu/mgba/issues/3852, it would be a nice addition.