When closing the inspector before the main window, we must take
care to sever all signal connections and weak refs, otherwise
things will go bad when the window is closed later.
This allows us to access otherwise hidden windows, which can be
useful to provide better debug information than would be available
through other means.
https://bugzilla.gnome.org/show_bug.cgi?id=736557
The entire UI is constructed with templates, so the wrapper
constructors are never called, except for gtk_inspector_window_new,
which gets called from the GTK+ code.
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