The template use in the inspector was not properly
disposing all widgets. gtk_widget_dispose_template
will only unparent widgets that have been named
as template children, so we need to make the toplevel
elements in the ui file named children, or manually
dispose them. This commit does the former.
Instead of iterating action groups manually,
just show what the action muxer provides. This
gives an accurate view of what actions are
available in a widgets context, and saves us
the trouble of juggling prefixes manually.
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.
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
