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/source-chooser.ui

@@ -33,6 +33,17 @@
                 <property name="tooltip-text" translatable="yes">the event cache, relative time and the interface's theme — the same dialog as the main window's menu, and the only place relative time can still change this run</property>
               </object>
             </child>
+            <!-- The help menu: the same three affordances the window's Help view carries, because a
+                 user who has not started yet is exactly the one who does not know how to start. Only
+                 the button is here — its rows carry a handler and a setting, so they are code, the
+                 same division every other row in this file follows. Beside `_Preferences`, because
+                 both act on the *program* rather than on a source. -->
+            <child type="start">
+              <object class="GtkMenuButton" id="help_menu_button">
+                <property name="icon-name">help-about-symbolic</property>
+                <property name="tooltip-text" translatable="yes">the manual, a guided tour after start-up, and the contextual help</property>
+              </object>
+            </child>
             <child type="end">
               <object class="GtkButton" id="cancel_button">
                 <property name="label" translatable="yes">_Cancel</property>