Provide a consistent interface to mouse-look mode across platforms

Mouse-look mode (where the mouse pointer is invisible, and moving the mouse directly controls some action in-game, like moving the camera) is a very common interaction mode in games, but is only clumsily supported in Panda. Panda should have a built-in function that makes this easier, and automatically works across all supported platforms. This should probably also be unified with the "pointer mode" vs. "raw mode" switch on GraphicsWindowInputDevice, which seems to mean a similar thing.