Make "mouse" events work on mobile devices

[almarklein]

Many interactions do not work on a touchscreen. This might mean we'd rename some of our standard widget events. So important to get this in before we enter "beta".

[Korijn]

Might I suggest Pointer Events? It would be nice to have a unified event system for touch, mouse and pen. See also:

• https://developer.mozilla.org/en-US/docs/Web/API/Pointer_events
• http://www.html5rocks.com/en/mobile/touchandmouse/

[Korijn]

https://developers.google.com/web/updates/2016/10/pointer-events?utm_source=feed&utm_medium=feed&utm_campaign=updates_feed

[almarklein]

Marking this for 0.5, since this might introduce backward incompatibilities (better do it sooner than later).