Fix pinch-to-zoom gesture

Having rotatable set to false (the default) previously disabled the TwoFingersTouchZoomRotateHandler completely, but we need it to stay enabled in order to still react to the zoom gesture.

Therefore, we now disable only the rotation by calling touchZoomRotate.disableRotation() explicitly.

Also, before this change users could still rotate the map via the keyboard shortcuts Shift+Left and Shift+Right; this is now explicitly forbidden when rotatable is false, too.

Lastly, most of the handlers will also disable pitch control when you disable rotation, but the TwoFingersTouchZoomRotateHandler does not. For consistency (and because it doesn't make a lot of sense to pitch without being able to rotate), we disable pitching too when rotation is forbidden.

Fixes #8 (closed).