Add key pressed to EventMotion constructor

[Ben Lambert]

Currently the EventMotion? user event only contains current location of the cursor in the window, should probably also contain which button is being pressed to trigger the EventMotion?.

[Ben Lippmeier]

This would be nice to provide, but GLUT doesn't tell us if the modifier keys change while the user is dragging. We could remember what modifers were pressed when the drag started, and return this information every time, but if any were released while dragging we'd be returning misleading information. It also seems possible to change what mouse button is being held while dragging, and GLUT doesn't report that either. I'm setting this as wontfix because GLUT doesn't give us enough information for it.