ui: the math editor's window, identity row and canvas are a .ui file

The second half of this plan row, and the half of that dialog which is template.
src/ui/math-editor.ui holds the window (its size and modality), the box around it,
the identity row -- the name and the unit -- and the body with the canvas in it.

What stays in code is the drawing: the canvas is a GtkDrawingArea the file *declares*
and the code draws, with the gestures that make it a graph editor, and the parameter
form follows which node is selected. The palette and the buttons go next.

The builder is held on `App` and cleared on the *next* opening rather than with the
window: the window belongs to the builder's object list, so freeing the builder under a
live window frees the window with it. That is the source form's pattern, and the probe's
close-then-reopen path is what exercises it -- `reopened: dialog=ok, nodes=2`.

The identity row is read back too, since a `.ui` can hold the widget but not the value:
`dialog build: ... name='math3' unit='' ...`, the entries having been filled from the
channel being edited. check_ui is at 12 ok, 0 failed.

src/ui_app.hpp

@@ -246,6 +246,10 @@ struct App {
   GtkWidget* math_toggle = nullptr;
   GtkWidget* math_revealer = nullptr;
   GtkWidget* math_list = nullptr;
+  /// The math editor's builder. The editor's window belongs to the builder's object list, so the builder is
+  /// held for as long as the window is up and cleared on the *next* opening rather than with the window --
+  /// freeing it under a live window frees the window with it. That is the source form's pattern.
+  GtkBuilder* math_dialog_ui = nullptr;
   GtkWidget* math_dialog = nullptr;
   /// One value label per measure, in `measures` order; rebuilt with the list.
   std::vector<GtkWidget*> measure_labels;