The behaviour of gtk_text_view_add_child_in_window() used to be
quite broken. It scrolled with the window during scrolling, then
jumped to the absolute position when the widget resized. Furthermore,
in 3.10 we broke the first feature, making it always be fixed.

The "proper" way to handle this is to always follow scrolling. This
is what the only user so far (gedit) wants, and if you want some
kind of overlay you should use GtkOverlay instead.

So, this changes the behaviour to something that is internally consistent
and works. I.e. all added widgets scroll with the textview as needed.

https://bugzilla.gnome.org/show_bug.cgi?id=711826