Introduce gtk_window_get/set_attached_to()

Introduce gtk_window_get/set_attached_to()

gtk_window_get/set_attached_to() is a new API that allows for windows to
be attached to a GtkWidget.
The attachment is a logical binding between the toplevel window and the
widget that generated it; this kind of information is currently used to
propagate style information from the widget to the window, but is also
useful e.g. for accessibility.

https://bugzilla.gnome.org/show_bug.cgi?id=666103