Scroll-bars are now hidden unless the cursor approaches them, in which case they
smoothly grow and become more & more visible. Note that since 0d309144020168e55,
scroll-bars are drawn on top of editor contents. There's no more jumping of
buttons when scroll-bars appear.
Technical notes:
* AZones are used to adjust scrollbars based on mouse movements
We may want to support screen level AZones if we want scrollbars to also
smoothly appear when approaching them from a different area.
I also plan to make further changes to AZones to clean up stuff a bit.
* Had to move AZone handling to a post ARegion init stage, since we need the
updated View2D data from there.
* View2D masks and scroller rectangles are now updated on every redraw. It's
cheap to do that though.
51efeb6834