Improved timeline zoom behavior

[erikari]

The zoom behavior on the Dopesheet timeline changed: the old behavior resulted in frame 0 being pushed off screen for any amount of zooming in, so you'd then need to scroll or right click drag to get frame 0 back.

We can improve it further by doing as we do now, but allow frame 0 to go off screen if required to keep the current frame on screen.

Reference thread:
http://esotericsoftware.com/forum/zooming-in-timeline-13677

[NathanSweet]

We can also allow the timeline to be zoomed using ctrl + right mouse drag, like in the viewport. This 1) gives stylus users a way to adjust dopesheet zoom besides the zoom slider, which is a bit sensitive since it is not very wide, and 2) gives a way to adjust dopesheet zoom from anywhere in the dopesheet (scroll wheel requires placing the mouse over the timeline).