This creates a new GtkTextViewChild that can manage overlay children at
given x,y offsets in buffer coordinates. This simplifies GtkTextView by
extracting this from GtkTextWindow as well as providing a real widget for
the borders.

With this change, we also rename gtk_text_view_add_child_in_window() to
gtk_text_view_add_overlay(). For those that were using
GTK_TEXT_WINDOW_WIDGET, they can use a GtkOverlay. It does not appear
that anyone was using GTK_TEXT_WINDOW_(LEFT|RIGHT|TOP|BOTTOM) for widgets
in this fashion, but that can be done by setting a gutter widget with
gtk_text_view_set_gutter(). We can make GtkTextViewChild public if
necessary to simplify this should it become necessary.

GtkTextViewChild will setup a CSS node of either "text" or "border"
depending on the GtkTextWindowType.

The old GtkTextViewChild has been renamed to AnchoredChild as it is only
used for widgets with anchors in the GtkTextBuffer. This also removes the
use of al...