Add a drag highlight to a row.
This is a helper function for implementing DnD onto a `GtkListBox`. The passed in row will be highlighted by setting the
gtk_state_flag_drop_active state and any previously highlighted row will be unhighlighted.
The row will also be unhighlighted when the widget gets a drag leave event.
| this |
a `GtkListBox` |
| row |
a `GtkListBoxRow` |