Panning controls and shortcuts for composer

This pull request improves navigation of compositions by:
- Adding a dedicated "pan" action for dragging around compositions
- Allowing panning compositions by holding and dragging with the middle mouse button
- Allow panning compositions by holding space key

This pull request replaces some earlier (inferior) versions. I'm happy with how this one works, although qt bug #3732 means the code isn't as tidy as it should be.

I'll send through the composer zoom in/out with mouse wheel, and a couple of other improvements to composition zooming as a seperate pull request.