Editor terrain: texture browser, undo, conversion from terrain files

- Terrain work mode shows a browser of the textures folder in place of the
  node bank; textures/materials can be dragged onto the palette entries
  (replacing the material), the material field and the water material field
- Ctrl+Z/Ctrl+Y for heightmap terrain changes: a brush stroke or an
  operation is one step of the editor history
- Ground conversion reads only the files of the terrain directives
  (.txtf with origin/rotate, or .btf without text), paints the chunks with
  the materials of the triangles, drops only the shapes of those files
- Fix: ground grids converted out-of-range floats to int, so whole-scene
  queries returned the triangles of a single cell (shifted chunks)
- Fix: terrain layers copied from textures already uploaded without their
  data stayed grey
- Fix: ImGui ID conflict between terrain tool radios and section headers
- Fix: layers of terrain .txtf files named with the scenery/ prefix and
  could not be saved
- Editor selftest commands for the terrain

scene/scenenode.cpp

@@ -429,7 +429,9 @@ bool
 shape_node::merge( shape_node &Shape ) {
 
     if( m_data.material != Shape.m_data.material
-     || m_data.lighting != Shape.m_data.lighting ) {
+     || m_data.lighting != Shape.m_data.lighting
+     || m_data.terrainfile != Shape.m_data.terrainfile ) {
+        // (shapes of different terrain files are kept apart, so the editor can tell which file the geometry comes from)
         // can't merge nodes with different appearance
         return false;
     }