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/editoruilayer.h

@@ -11,6 +11,7 @@ http://mozilla.org/MPL/2.0/.
 
 #include "application/uilayer.h"
 #include "application/editoruipanels.h"
+#include "application/planpanel.h"
 
 namespace scene
 {
@@ -38,11 +39,15 @@ class editor_ui : public ui_layer
 	float getSpacing();
 	void toggleBrushSettings(bool isVisible);
 
+  protected:
+	void render_menu_contents() override;
+
   private:
 	// members
 	itemproperties_panel m_itempropertiespanel{"Node Properties", true};
 	functions_panel m_functionspanel{"Functions", true};
 	nodebank_panel m_nodebankpanel{"Node Bank", true};
 	brush_object_list m_brushobjects{"Brush properties", false};
+	plan_panel m_planpanel{"Plan", false};
 	scene::basic_node *m_node{nullptr}; // currently bound scene node, if any
 };