OpenMW-CS: Terrain shape editing
After transient change support (#4840 (closed)), implement a mode to edit terrain height data. Changes done to terrain shouldn't be immediately done to data, but rather to temporary transient data which is moved to actual data when the edit button is released.
The height editing mode should have various brushes and modes in order to accommodate artistic work. ESM file format has limits for the land heights, and cell edges have to be properly handled in all cases.
User setting should be implemented for the behavior when editing adjancent cells or cell edges. User should be able to choose whether to edit all cells, or only selected cells. When editing only a single cell, no changes should be done to adjancent cells. This requires that vertexes at the cell edge are not changed, if user doesn't want to alter the data also at the adjancent cells.
Brushes: vertex, square, circle, custom Tool modes: Drag heights, raise heights by painting, lower heights by painting, smooth, flatten. Later, adding bumps or other "intelligent" shapes could be implemented.