Check if shortcuts are already in use

[anewuser]

puNES should prevent users from accidentally setting the same shortcut for different actions:

Windows 10
puNES 64-bit WIP (D3D9)

[punesemu]

Thanks for the tip. Now I'm busy rewriting all the mappers and a part of the emulator core (WRAM_Rework branch), as soon as I've finished this monumental work I'll dedicate myself to the various issues.