Hide rotation controls on touchscreen devices

For touchscreens, it makes far more sense to use the native gestures
Allow arbitrary rotation to be used with OpenSeadragon

The touchscreen experience feels far more natural when arbitrary
rotation is allowed, which works well with a rotation event driving
the set-rotation viewer event. Unfortunately OpenSeadragon does not
yet support free rotation[1] so we'll round the angles to the 
nearest 90° for the seadragon view.

1. https://github.com/openseadragon/openseadragon/issues/194
Don't reinitialize views which are already open

This avoids a bit of overhead if external JavaScript triggers the
same open view event repeatedly