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_widgets.cpp

@@ -37,6 +37,13 @@ GtkBuilder* ui_load(App& app, const char* resource) {
 /// the whole side panel. Scoped to `.udpscope-compact` so it cannot reach any other
 /// button in the window.
 void install_css() {
+  // Idempotent, because it is no longer installed from one place: the source chooser runs before the
+  // application (it draws a menu built from the same `menu_button()` rows) and has to have the
+  // stylesheet before it shows them, while `on_activate()` still installs it for every run. A second
+  // provider would be a second copy of the same rules on the display.
+  static bool installed = false;
+  if (installed) return;
+  installed = true;
   static const char kCss[] =
       "row.udpscope-compact { min-height: 0; padding: 1px 0; }\n"
       // A popover menu row: Adwaita gives a button a 34 px minimum height and 8 px of padding,