OpenMW-CS: Selecting terrain in "Terrain land editing" should support "Add to selection" and "Remove from selection" modes

Currently, the primary-select key is hardcoded to a function similar to the "Select only" option for the instance-drag selection. Likewise, the secondary-select key is hardcoded to the "Invert selection" equivalent.

Even if this behaviour is intended and the solution used by the instance-drag selection is only temporary, it is totally unexpected and confusing for the user. For the time being, we should just mimic the instance-drag selection here.