Clean tablet support implementation

[bloerwald]

@SteffMC reported on 2017-06-07

Central ui for tablet settings. Fast toggle of tablet and normal mode. Testing if it works like planed.

I plan to integrate the tablet settings (if turned generally on in settings) into a new control bar at the bottom of the 3d editor. There will also go daytime and move speed settings into.

[bloerwald]

The issue of needing a quick toggle has come up recently on Discord as well.