initial proof of concept for space navigator 3d mouse support

[justbuchanan]

This is only partially working and isn't ready for a full review yet. Sending this now to get feed back on a couple things:

• Assuming I clean this up and get it working well, is this something you'd like to merge into cq-editor?
• Any suggestions on where this code should live? Is occt_widget.py the right place for it?

Things left to be done:

• there seems to be some lag / delayed effect. When you stop moving the mouse, the view keeps changing for a bit
• see if we can use the pyspacemouse callback API rather than periodically polling for new mouse values
• handle mouse connects/disconnects while cq-editor is running. Right now if you plug in the mouse after running cq-editor, you'd have to restart it to get it to notice the mouse. Since this requires periodically polling for the mouse, we'll want it to be a default-off toggle in preferences.
• only pan and zoom are supported - more work is needed for rotation
• the math / scale factors for mapping mouse values to pan/rotate/zoom are pretty terrible
• ?

Addresses #412