View2D - Zoom + More Scrollbar work

View2D - Zoom + More Scrollbar work

* Added basic (non-modal) zoom operators that use a uniform scale factor, with zoom centered using the view center as scaling point. Use Scrollwheel up/down and Pad +/- to use this.

* Added back the 'button'/bubble for the scrollbars. I've added dark lines on either end of it for some later work on zooming widgets.
This is not the final form they'll take. I still need to decide how to handle those scrollbars which act as grid-markers too (showing timescale, etc.), before trying to integrate that with some fancy scrollbar drawing (rounded, etc.)

Assorted changes:
* Moved vertical scrollbar for Outliner to right hand side
* Made Timeline use standard scrollbars, and turned on various clamping options
* Fixed ortho-matrix corrections for scrollbars, and added pixel offsets
* Made Timeline markers sit more snugly on the scrollbar. They were a bit far out...
* Fixed memory leak with view2d keymaps not being freed when Blender exited