Editor: fold the plan library into the tree as modules, drop its own name

The track layout library came in as a subproject of its own -- its own
CMakeLists, its own install and export rules, its own include tree -- which only
made the build harder to follow now that everything else is a module. It is part
of the editor, so it lives where the editor lives.

- editor/plan_*.cppm / plan_*.cpp: the fifteen interface units and twelve
  implementation units, flat in editor/ beside editorTerrain and orthophoto, with
  the plan_ prefix keeping them clear of what was already there (editor/turnouts
  is the preset catalogue, so the geometry is plan_turnout_geometry).
- modules are named eu07.editor.plan_*, like every other module in the tree.
- namespaces move under the editor's own: editor::plan, editor::plan::geometry,
  ::io, ::render. The library's old name is gone from the sources and the docs.
- every moved file carries the MPL header the rest of the tree carries; it had
  none. version.hpp went with the subproject: macros only, and unused.
- the umbrella header is dropped -- nothing imported it.
- the unit tests survive the fold: editor/tests, behind EU07_EDITOR_PLAN_TESTS
  (off by default), compiles the interfaces a second time into a small library of
  its own, because a test can only import modules from a target it links and
  linking the game would drag the whole simulator in. The code under test needs
  nothing but a compiler. 7/7 pass.
- a switch may now be written with two dashes as well: --editor beside -editor.

Builds with clang 22 (-DWITH_BETTER_RENDERER=OFF); -editor and --editor both come
up in the editor on empty ground.

application/application.cpp

@@ -1176,6 +1176,13 @@ int eu07_application::init_settings(int Argc, char *Argv[])
 
 		std::string token{Argv[i]};
 
+		// a switch may be written with one dash or two: both spellings are common on the command
+		// line and neither says anything different, so the second dash is simply dropped here
+		if (token.size() > 2 && token.compare(0, 2, "--") == 0)
+		{
+			token.erase(0, 1);
+		}
+
 		if (token == "-s")
 		{
 			if (i + 1 < Argc)
@@ -1199,7 +1206,8 @@ int eu07_application::init_settings(int Argc, char *Argv[])
 		{
 			std::cout << "usage: " << std::string(Argv[0]) << " [-s sceneryfilepath]"
 			          << " [-v vehiclename]"
-			          << " [-editor]" << std::endl;
+			          << " [-editor]"
+			          << "  (every switch also takes a double dash: --editor)" << std::endl;
 			return -1;
 		}
 	}