Mnemonics need to be triggered with help from the controllers (who
determine the modifiers). Support for that has been added, too.
Mnemonics do not use this yet though.
And use it.
I just added it to GtkWidget just to show that I can.
The real reason I want it is for gamepad/joystick triggers
in games, so that it becomes possible to select 2 different
triggers (gamepad and keyboard) for the same shortcut.
