Proposed UX for the exploded view

[SylvainCorlay]

I propose to make the exploded view toolbar button a toggle.

The exploded view factor number input could be set either in a separate dropdown in the toolbar (at first), or in a text input overlayed on the view.

On a separate note, maybe there should be a visual indication in the view that the exploded view is not the normal position of objects - and how to leave this view, in case someone does not know how to set it back to normal.

[martinRenou]

Closing as duplicate of #443