ui: the source chooser's window is a .ui file, and its pages an AdwViewStack

The last hand-built UI outside ui.cpp. The window, its header bar, its two pages and the
four buttons that are about the chooser rather than about a source now live in
src/ui/source-chooser.ui; the rows stay in code because a source row and a waveform row
are data, rebuilt whenever the known hosts or the recent files change.

The pages were a GtkNotebook and are an AdwViewStack under an AdwViewSwitcherBar, which
is what current libadwaita asks for and the same information without a tab strip. The
three actions moved from a button row at the foot into the header bar, the shape
source-form.ui already uses -- and deliberately without receives-default, since Enter
must keep activating the selected row.

Nothing carries a value: the tooltips and labels moved as they were, the handlers are
the same functions, and the switch is by page *name* rather than by index.

The probe reads the frame back -- `[sources] chooser ui: page='sources' title='Sources'
icon=yes page='waveforms' … switcher_wired=1 buttons=[prefs=1 new_host=1 open=1
cancel=1]` -- so a .ui that lost an id is named instead of opening a window with a hole
in it, and `check_ui` type-checks the file against the GIR (7 ok, 0 failed).

Two things the checks caught while doing it: the new frame dropped the call that fills
the waveforms page (the unused-function warning), and `AdwViewStackPage` is a GObject
that *wraps* a child rather than being one, so reading the pages needs
adw_view_stack_get_page().

src/ui/udpscope-ui.gresource.xml

@@ -13,6 +13,7 @@
     <file preprocess="xml-stripblanks">preferences.ui</file>
     <file preprocess="xml-stripblanks">help.ui</file>
     <file preprocess="xml-stripblanks">source-form.ui</file>
+    <file preprocess="xml-stripblanks">source-chooser.ui</file>
     <file preprocess="xml-stripblanks">main-window.ui</file>
     <file preprocess="xml-stripblanks">cursors-panel.ui</file>
   </gresource>