Fixes a number of glitches, e.g. the sidebar disappearing when selecting an
asset or wrong AZones (the little chevrons to indicate a hidden region).
There were a couple of issues:
* Execution region was created, but not used.
* If an execution region already existed when refreshing the area, it was
tagged as hidden, not removed.
* The sidebar was always set to be hidden on refreshes.
* When toggling from Asset Browser to File Browser as regular editor (i.e. not
opened temporary via Ctrl+O or such), the sidebar region wasn't removed.
Adresses T83644.
3cc7e2ad9d