Link zoom slider to view width

[matthewturk]

The zoom slider is currently not connected to any traits, so it's hard to do two-way binding. But, we could fix this in a straightforward way by either linking it to a trait (and then observing that trait) or by providing a handle that updates it.

Regardless, we should have it set so that the linkage between the zoom slider and the zoom from the mouse wheel is set.