Rework polygon/polyline dragging to fix perf problems #13011
Description
Related to #12959
Problem is that too much work happens behind the scenes when you drag:
- Recalcing mission items
- Recalcing Terrain view
- ...
All of this can end up taking a bunch of time making drag very laggy.
How it should work is that while dragging the UI should switch to a drag mode which only shows the polyline/polygon bounds visually but does not update all the other internals. And then once drag stops the internals like expensive Terrain profile recalc can happen.