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.

CMakeLists.txt

@@ -173,6 +173,7 @@ set(SOURCES
 "input/editormouseinput.cpp"
 "application/editoruilayer.cpp"
 "application/editoruipanels.cpp"
+"application/planpanel.cpp"
 "application/scenarioloadermode.cpp"
 "application/scenarioloaderuilayer.cpp"
 "scene/scenenodegroups.cpp"