Adds child at a fixed coordinate in the `GtkTextView`'s text window.
The xpos and ypos must be in buffer coordinates (see [[email protected]_iter_location] to convert to
buffer coordinates).
child will scroll with the text view.
If instead you want a widget that will not move with the `GtkTextView` contents see `GtkOverlay`.
| this |
a `GtkTextView` |
| child |
a `GtkWidget` |
| xpos |
X position of child in window coordinates |
| ypos |
Y position of child in window coordinates |