[regression] MirEvent 2.0 has no replacement for mir_motion_tool_type_eraser

It's unclear that mir_motion_tool_type_eraser makes sense or is required. Originally we copied it from android thoughtlessly (this isn't slander, I did it, it was thoughtless). Qt doesnt feature it.

When writing the new input event headers I decided since Qt doesn't feature it its probably not required and perhaps better expressed as some sort of modifier state on the touch.

Incomplete. I need to check the evdev stream. If it is recognised separately to the stylus then it is indeed a new tooltype.

We don't want MirEvent 2.0 to lose features we already have in MirEvent 1.0 ...

Confirmed in evtest ... the eraser is reported by evdev as a different tool "BTN_TOOL_RUBBER" vs the other end which is "BTN_TOOL_PEN".

So the eraser is indeed a different tool type.

Syncing task from Mir.