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.