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/application.cpp

@@ -1155,8 +1155,8 @@ int eu07_application::init_settings(int Argc, char *Argv[])
 
 	if (Global.editor_startup && Global.SceneryFile.empty())
 	{
-		// edytor potrzebuje wczytanego świata, ale niekoniecznie czegokolwiek w nim - pusta sceneria daje
-		// sam region, oświetlenie i zegar, bez taboru i rozkładów
+		// the editor needs a loaded world, but not necessarily anything in it - an empty scenery gives
+		// just the region, the lighting and the clock, with no rolling stock and no timetables
 		Global.SceneryFile = "pusta.scn";
 	}