GtkPopover:pointing-to: GdkRectangle instead of cairo_rectangle_int_t

GtkPopover:pointing-to: GdkRectangle instead of cairo_rectangle_int_t

cairo_rectangle_int_t was replaced by GdkRectangle in commit
552c29b488ecd7bcc3303dd5514ce6dbfff04437, but the type of the pointing-to
property was not changed.

To avoid breaking old code that sets or gets the property with a GValue
of type CAIRO_GOBJECT_TYPE_RECTANGLE_INT, transformation functions between
CAIRO_GOBJECT_TYPE_RECTANGLE_INT and GDK_TYPE_RECTANGLE are registered on
the first call to gdk_rectangle_get_type().

https://bugzilla.gnome.org/show_bug.cgi?id=723394