Use pointer events

This pull request bases the internal event system of OpenLayers on [pointer events](https://dvcs.w3.org/hg/pointerevents/raw-file/tip/pointerEvents.html).

So far, there were separate interactions for touch and mouse events. Now, the interactions only receive pointer events. These pointer events are generated from a `PointerEventHandler`, which listens to mouse events, touch events or real pointer events (on IE 10 and 11). The code is based on the pointer events polyfill of the [Polymer library](https://github.com/Polymer/PointerEvents).

All existing interactions have been ported to pointer events. The modify and draw feature interaction have been adapted by @ahocevar.

Also thanks to @fredj and @elemoine for the feedback!

Closes  #1099