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/ui_tour.cpp

@@ -342,6 +342,16 @@ gboolean tour_autostart(gpointer user) {
   return G_SOURCE_REMOVE;
 }
 
+gboolean tour_start_requested(gpointer user) {
+  App* app = static_cast<App*>(user);
+  if (app->window == nullptr || app->tour_overlay == nullptr) return G_SOURCE_REMOVE;
+  // Not `tour_auto_started`: the chooser's entry is the user asking for the tour before the window
+  // existed, and the reading has to say which of the two started it.
+  app->tour_requested = true;
+  start_tour(*app);
+  return G_SOURCE_REMOVE;
+}
+
 // --- the dump's readings ----------------------------------------------------
 
 namespace {
@@ -381,10 +391,11 @@ void capture_help_state(App& app) {
   char layer[512];
   std::snprintf(layer, sizeof(layer),
                 "contextual=%s hints=%d ids={%s} icon=dialog-question-symbolic(%s); tour=%s steps=%d "
-                "step=%d auto=%d",
+                "step=%d auto=%d requested=%d",
                 app.settings.contextual_help ? "on" : "off", hints, ids.empty() ? "" : ids.c_str(),
                 icon_ok ? "ok" : "MISSING", app.tour_running ? "running" : "idle",
-                tour_step_count(app), tour_step_index(app), app.tour_auto_started ? 1 : 0);
+                tour_step_count(app), tour_step_index(app), app.tour_auto_started ? 1 : 0,
+                app.tour_requested ? 1 : 0);
   app.dbg_context_help = layer;
   app.dbg_tour_steps = tour_step_count(app);
   app.dbg_tour_step = tour_step_index(app);