GtkTextHandle: Look up for the first child of a scrolled window found

GtkTextHandle: Look up for the first child of a scrolled window found

Text handles use to connect to the first GtkScrollable up the hierarchy
so they can be repositioned when scrolling. It makes more sense to look
up the first child of a GtkScrolledWindow, it must be an scrollable too,
and will be the scrollable that can actually change the position of the
text handles.

https://bugzilla.gnome.org/show_bug.cgi?id=761676