Avoid unwanted drag cancel animations

GTK+ is now being more careful about dealing with tabs
disappearing in the middle of a drag - which is exactly
what the current gedit code appears to be doing, from
the GtkNotebook viewpoint. To avoid this, use a new function
that was introduced for this purpose in GtkNotebook.

https://bugzilla.gnome.org/show_bug.cgi?id=744443