i3: Disable all shortcuts when gaming

I was playing Biomutant and accidentally made it close several times because of the key bindings the game used in busy fight situations.

The use of Q W A S D E, Alt and Shift made me slip to the super key quite often. Super+Shift+Q in my case kills an application. Not so good while fighting.

Solution: modes.

When in a mode, new key bindings apply, which basically disables all usual bindings for the time being.

So I added a mode "play_game" with no bindings except the one to exit the mode (which is the same as how to enter).

I used the Scroll-Lock key which is very far from any key I might press.

bindsym Scroll_Lock mode "play_game"

mode "play_game" {
        bindsym Scroll_Lock mode "default"
}