Trackpad-specific input event support

[thatcomputerguy0101]

This adds support for pinch and rotation gesture events, as exposed from winit on MacOS platforms. Additionally, this associates ctrl-scrolling with the pinch gesture unless actual gesture events are detected since some Windows trackpads expose pinch information as scrolling with the ctrl key held. This somewhat breaks any custom control systems that expect pinch to zoom until they are updated with the new pinch event. However, since key modifiers are not made accessible to the camera control interface, any old handlers are still accessible by scrolling vertically.

[asny]

That's a nice feature to add 👌

[thatcomputerguy0101]

Instead of remapping events, I added the capability to execute different camera actions depending on the state of the modifier keys. This removed the Copy trait from CameraAction and CameraControl due to Box now being involved, but standard uses of those two types outside of the three-d crate should not be affected.