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

application/editoruipanels.cpp

@@ -934,6 +934,14 @@ void nodebank_panel::render()
 	{
 		ImGui::SetNextWindowSizeConstraints(ImVec2S(size_min.x, size_min.y), ImVec2S(size_max.x, size_max.y));
 	}
+	if (m_standin)
+	{
+		// the same window, so it keeps its place in the dock
+		if (ImGui::Begin((m_standintitle + "###" + name()).c_str(), nullptr, flags))
+			m_standin();
+		ImGui::End();
+		return;
+	}
 	auto const panelname{(title.empty() ? name() : title) + "###" + name()};
 
 	if (true == ImGui::Begin(panelname.c_str(), nullptr, flags))