The sidebar in the object page was using up more space than it deserves.
Replace it with a combo box in the subheader. Now that we have more space,
put the CSS node tree and the style properties in the same page again.
These functions, while added for use by the GTK inspector, are generally
useful to applications that need to resolve what action groups are
available to a particular GtkWidget.
https://bugzilla.gnome.org/show_bug.cgi?id=741386
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.
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
