Normalize zoom slider's position for the zoom range

With there being a linear mapping between the slider's position and the zoom level, the middle position of the zoom slider has always *not* matched the zoom level of `1`.  This was not very easily perceivable with the old zoom range of 0.3 and 1.5 (only slightly off-center), but with the new zoom levels from `0.25` to `4` this effect is highly pronounced.

This PR makes more natural -- the middle corresponds to `1`, the leftmost to `0.25`, the rightmost to `4`.

Fixes #440.