This PR is a different solution to what @gudnimg found in PR#233 / PFW-1404.
The benefit of this approach is the fact, that the button press event is generated when the button is pressed and not after it has been released.
The downside is obvious:
CPUFLASH: +28B
RAM: +1B
Sets the UI module into a mode when the printer is in charge of processing the buttons (from all sources).
That means the MMU will detect its buttons but it will not react upon them.
This mode is important for error recovery when the printer needs to do some stuff before the MMU (like preheating the nozzle).