VolumeViewer: Updating a panel.zoom property triggers the zoom event

I used the same dirty checking mechanism as in [the surface viewer](https://github.com/aces/brainbrowser/blob/f61329c3a4311b0864814932f529432ca1f2fe54/src/brainbrowser/surface-viewer/modules/rendering.js#L419-L423), except that the `old_zoom_level` is stored as a "private" (non-documented) property of a panel.

fix #122