Add a force quit function

[MTFT-Games]

A method of forcing a game to quit back to menu should be implemented.
This will likely be done when both menu buttons are pressed and held for several seconds.
The frontend seems to still receive input even when its not focused so it should be able to recognize the input with a game open. The frontend should then command the backend to kill the currently running game, either by keeping track of the pid of the last launched process or it might be able to kill all bwrap instances.