Add a track layout drawing board to the editor

A "Plan" panel, opened from Track layout in the editor's menu bar, draws a
niweleta the way the web frontend did: click to lay chained straights, drag
a corner to move it, and pick a fit for each gap between them. The geometry
is entirely maj0sted's - the panel gathers the input, hands the document to
solve_project and paints the rails that come back, coloured by whether they
came from a straight, an arc or a clothoid. Scenery nodes are untouched.

A fit the library cannot produce is simply absent from its answer, so the
gap is labelled as not fitting rather than left looking as though its
parameters took effect. Save/load go through the library's own document
format, so a project written here opens in the same state.

Also translates the comments added with -editor into English, matching the
files they sit in.

application/scenarioloadermode.cpp

@@ -60,8 +60,8 @@ bool scenarioloader_mode::update() {
 	// TODO: implement and use next mode cue
 
 	Application.pop_mode();
-	// przy starcie do edytora symulacja w ogóle nie rusza: nie ma trybu prowadzenia pod spodem, więc nie
-	// liczy się fizyki, AI ani rozkładów - zostaje sam wczytany świat do obrabiania
+	// starting into the editor the simulation never runs at all: there's no driver mode underneath, so
+	// no physics, no AI and no timetables - just the loaded world to work on
 	Application.push_mode(
 	    Global.editor_startup ?
 	        eu07_application::mode::editor :