Optimise for Touch Support

[mirkofetter]

The library to some extend already works with touch, but some things are not yet supported.
For example scaling with a zoom gesture (ZoomEvent) or dragging connectors with the finger does not work. Would be really nice to make this work for interaction on big touch screens.

[rmfisher]

This would be nice to have but it's not something I'll be able to implement myself in the near future.