This makes it so that regions only needed when the file browser is
invoked as an operation (e.g. Ctrl+O rather than a regular editor) are
lazy created then, and removed if the file browser is changed into a
regular editor then (e.g. Ctrl+O over regular file browser editor ->
Cancel).
That should remove some troublesome assumptions and makes versioning
redundant.
It also fixes the issue of an empty execute region at the bottom after
cancelling a file operation invoked from a regular file browser editor.
d1cc340e56