help: default the contextual layer off, and carry the help into the chooser

The contextual help was on by default, so a first run met a window with a "?"
beside every surface it had not asked to have explained; `contextual_help` now
defaults to false and the layer is asked for from the Help menu, from the
chooser's new help menu, or with Ctrl+H.

The source chooser runs before `App` exists, which makes it the window a user
who does not know how to start is looking at, so it now carries the same three
help affordances the window's Help view does: Documentation (the manual is
another process and needs nothing of the app), Enable contextual help (a row
whose check *is* the setting, written to the file the app reads a moment
later), and Guided tour. The tour is drawn on the app's own window, so the
chooser can only record the wish: it sets `Config::tour_pending` -- the value
`main()` owns and hands to both `choose_source()` and `run_ui()` -- and
`on_activate()` starts it once the window is up and laid out, through its own
callback so the readings can tell a request from the first-run auto-start.
The rows are the window's own `menu_button()`/`menu_toggle()`; `install_css()`
therefore runs from `choose_source()` (it is idempotent now) so the popover
gets the `.udpscope-menu` rules before it is shown.

Opening the manual could also fail with a message that blamed the handler
whatever was missing. `help_installed_path()` now resolves `help:<id>` the way
the specification does and the failure alert says which of the two it is: with
the set installed it names the viewer, without it it names the identifier, the
expected installed path and the install command. `show_help()` gained an
optional reporter, which `UDPSCOPE_PROBE_HELP_MS` uses for a new reading --
`[help] launch attempted= done= ok= error='' installed=` -- since a successful
launch leaves no window of ours to observe.

Tested: clean --clean-first build, three warnings as documented; test-ui 13/13
and test-help pass; no Gtk-CRITICAL in the help, helpui and chooser runs.

src/config.hpp

@@ -59,6 +59,13 @@ struct Config {
   /// live source. File mode and source mode are exclusive -- there is no acquisition behind a file,
   /// and the chooser's Waveforms page sets this rather than the connection fields.
   std::string open_waveform;
+
+  /// The source chooser's "Guided tour" entry. The tour is drawn on the application's own window,
+  /// which does not exist while the chooser runs, so the request rides on the one value `main()`
+  /// owns and hands to both the chooser and the front end (`open_waveform` and the preferences'
+  /// relative time cross the same way) and the window starts the tour once it is up and laid out.
+  /// It is *not* a preference: it describes this request and nothing else, so nothing persists it.
+  bool tour_pending = false;
 };
 
 /// Parses argv. On `--help` prints usage to stdout and exits(0); on a bad