Fix undo in sheets

[ocs]

When a sheet is active (tested in 2.14.0 (2922e78) with the Source list/editor, would most probably apply to other sheets too), Undo is disabled. That's somewhat confusing and inconvenient.

Open the Source list/editor sheet, make some changes, change your mind. Open the Edit menu, oops: Undo is disabled!

Nevertheless, soon as you close the Source list/editor, Undo comes back and actually works

It would be desirable that Undo is available and works all the time, regardless of the current application state.

Note before you start fixing this, do please check also the use-panels request, which (if accepted) would very probably fix the problem automatically as a side-effect with no effort at all.