ui: the measurements panel's frame is a .ui file

The panel's shape -- the title row with its add button, the one statistics switch
the whole panel shares, and the box the readings go into -- now lives in
src/ui/measures-panel.ui. The readings stay in code: each row is a measurement a
user placed, rebuilt whenever the set or the mode changes.

A faithful move, not a redesign: the same margins, spacing, labels, tooltips, flat
classes and insensitive Reset button, the same two lambdas on the same two signals.

The frame is read back through the dump line, which grew the two fields that can
disagree with the code that drives them -- `measures pane: n=4 revealed=0
marks_shown=6 pane_rows=4 pane_w=0 pane_h=0 switch_on=1 reset_sensitive=1`. In that
reading pane_rows=4 is the four readout rows having landed in the frame's
`measures_list`, and switch_on/reset_sensitive are the switch widget and the Reset
button following measure_stats_on, which the probe set through the same signal a user
would. `check_ui` type-checks the file (8 ok, 0 failed) and the build stays at four
warnings.

src/ui/udpscope-ui.gresource.xml

@@ -16,5 +16,6 @@
     <file preprocess="xml-stripblanks">source-chooser.ui</file>
     <file preprocess="xml-stripblanks">main-window.ui</file>
     <file preprocess="xml-stripblanks">cursors-panel.ui</file>
+    <file preprocess="xml-stripblanks">measures-panel.ui</file>
   </gresource>
 </gresources>