app: Implement canvas support for touchpad gesture rotation by pinch

This MR implements support for touchpad pinch gesture to control rotation within the canvas (just like Shift and middle mouse button click does). Internally it works similarly to the previous MR to add zooming (!401 (merged)).

Coarse rotation works via Control keyboard modifier. Note that pinch gesture will activate both zooming and rotation, so it's currently not possible to do only rotation or only zooming.