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/settings.cpp

@@ -84,9 +84,10 @@ bool save_settings(const Settings& settings) {
   out << "event_cache_max = " << settings.event_cache_max << "\n";
   out << "relative_time = " << (settings.relative_time ? "true" : "false") << "\n";
   out << "theme = \"" << settings.theme << "\"\n";
-  // The two help keys are the *app's* memory rather than the dialog's: the contextual-help toggle in
-  // the Help menu writes the first, and the first-run tour writes the second. `read_rows()` in the
-  // preferences starts from `load_settings()`, so a dialog save carries them through untouched.
+  // The two help keys are the *app's* memory rather than the dialog's: the contextual-help rows --
+  // the Help menu's and the source chooser's own -- write the first, and the first-run tour writes the
+  // second. `read_rows()` in the preferences starts from `load_settings()`, so a dialog save carries
+  // them through untouched.
   out << "contextual_help = " << (settings.contextual_help ? "true" : "false") << "\n";
   out << "tour_seen = " << (settings.tour_seen ? "true" : "false") << "\n";
   out.flush();