OpenMW-CS: Terrain selection is not updated when undoing/redoing terrain changes

To reproduce:

1. Open an arbitrary cell in the scene view.
2. Under "Edit Mode" in the control bar, select the "Terrain land editing" mode.
3. Under "Change brush settings", select any brush (ideally, one with bigger dimensions).
4. Choose "Height (drag)" in the brush's dropdown menu.
5. Use your primary-select key to select part of the visible terrain.
6. Use your primary-edit key to raise at least part of the terrain which is currently selected.
7. Release the key and undo the change.
8. The selection stays at the altered terrain level and is not reset.

(This is also true for redoing changes and consecutive undo/redo actions.)