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/udpscope-ui.gresource.xml

@@ -18,6 +18,7 @@
     <file preprocess="xml-stripblanks">cursors-panel.ui</file>
     <file preprocess="xml-stripblanks">measures-panel.ui</file>
     <file preprocess="xml-stripblanks">math-panel.ui</file>
+    <file preprocess="xml-stripblanks">math-editor.ui</file>
     <file preprocess="xml-stripblanks">trigger-panel.ui</file>
     <file preprocess="xml-stripblanks">signals-panel.ui</file>
   </gresource>