input: use keyboard consumers to seperate cell and overlay logic

[Megamouse]

Currently it is possible to initialize a basic keyboard handler in the overlay input loop, ignoring cellKb.
This can potentially cause bugs or unexpected behaviour if a game uses cellKb (like having key repeat active by mistake).

• Adds consumer subscription model to the basic keyboard handler.
• cellKb and the overlay input loop now each have their own subscription.
• This allows the overlay input loop to use key repeat by default without influencing cellKb behaviour.
• Properly ignore basic keyboard input in the osk if the pad handler is set to keyboard and pad input is disabled
• Added the caps lock and the meta key (windows key) to the basic keyboard handler
• Added opencv to gitignore for undisclosed reasons

Supersedes #15501
Thanks for finding these issues @cipherxof