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_probe.cpp

@@ -389,6 +389,12 @@ gboolean math_probe(gpointer user) {
       const std::size_t built = ed->nodes.size();
       const double gain_read = ed->nodes.empty() ? 0.0 : ed->nodes.back().gain;
       const double offset_read = ed->nodes.empty() ? 0.0 : ed->nodes.back().offset;
+      // The identity row, which the file cannot hold values for: the entries were filled from the channel
+      // being edited, so reading them back is what shows the row is the same one the code writes.
+      const std::string name_read =
+          ed->name != nullptr ? gtk_editable_get_text(GTK_EDITABLE(ed->name)) : "-";
+      const std::string unit_read =
+          ed->unit != nullptr ? gtk_editable_get_text(GTK_EDITABLE(ed->unit)) : "-";
       on_math_apply(nullptr, ed);  // frees the dialog: nothing below may touch `ed`
       const std::size_t saved = a->math.empty()
                                     ? 0u
@@ -397,9 +403,10 @@ gboolean math_probe(gpointer user) {
           a->math.empty() ? -1 : a->math[static_cast<std::size_t>(index)].output;
       std::fprintf(stderr,
                    "[math-probe] dialog build: nodes=%zu, output after add=%d (want %zu); "
-                   "reading back gain=%.2f offset=%.2f; saved nodes=%zu output=%d\n",
-                   built, output_after_add, built - 1, gain_read, offset_read, saved,
-                   output_saved);
+                   "reading back gain=%.2f offset=%.2f; name='%s' unit='%s'; "
+                   "saved nodes=%zu output=%d\n",
+                   built, output_after_add, built - 1, gain_read, offset_read, name_read.c_str(),
+                   unit_read.c_str(), saved, output_saved);
     }
   }
   // Closed and reopened: `gtk_window_destroy()` does not emit `close-request`, so a dialog closed