Add yet another tab for showing information about a widget that
does not quite fit into any of the other tabs (not a property,
not style information, etc...).
For now, we show the widget state, as well as the default and
focus widget for windows.
In contrast to the flashing, where we blink the widget a
few times, this is explicitly turned on and off.
It will be used for indicating widgets that are part of
a size group, in the next commit.
Show the actions that are added to GtkApplication and
GtkApplicationWindows, as well as action groups that are
inserted elsewhere with gtk_widget_insert_action_group.
https://bugzilla.gnome.org/show_bug.cgi?id=730095
Moving the inspector into libgtk lets use reuse internals without
having to add public API for everything or inventing awkward private
call conventions.
https://bugzilla.gnome.org/show_bug.cgi?id=730095
