Allow viewport panning with space bar + drag

[deactivated]

This patch allows you to pan the viewport by holding the spacebar and dragging with the left mouse button.

The behavior mimics Photoshop and frees the right mouse button for context menus and component rotation (as in EAGLE).

[davidcarne]

Unfortunately, I can't merge this yet. Event delivery seems to occur very differently on Linux. I do like the control methodology though, so I'll probably end up adopting the control scheme; but I need to figure out how to get it to work right on linux first.