GtkWidget: Add gtk_widget_insert_action_group()

GtkWidget: Add gtk_widget_insert_action_group()

This allows adding a GActionGroup with a given name at an arbitrary
point in the widget tree.

This patch also adds an internal _get_action_muxer() API.  Calling this
will create a GActionMuxer associated with the widget.  The parent of
the muxer will be the muxer of the widget's conceptual parent.  For
non-menus, that is the normal parent.  For menus, it is the attach
widget.

In this way, we end up with a hierarchy of GActionMuxer that largely
reflects the hierarchy of GtkWidget, but only in places that the action
context has been requested.  These muxers are the ones on which the
inserted actions groups are installed.

A following patch will add a user of this API.