View2D - Another WIP commit

View2D - Another WIP commit

* Start of basic scrollbar drawing. This will be improved. Only Outliner shows these for now, as although the Timeline should show them, the old files didn't have them turned on.

* Tidied up the view-panning operator
- Fixed naming convention
- Added user-adjustable properties (deltax, deltay in screenspace)

* Added ctrl-scrollwheel (horizontal) and shift-scrollwheel (vertical) scroll operators. These use the view-panning code too.
Unfortunately, I haven't been able to figure out why the WHEELMOUSEDOWN events don't seem to be triggering the operators!