Add an explicit GtkDragSource object, and move the source-side
DND signals here. The object can be used on the spot, by calling
gtk_drag_source_drag_begin() yourself, or for static drag-source
configuration, by calling gtk_drag_source_attach(). In the latter
case, we set up and conect a drag gesture to trigger the drag_begin
call.
Remove all the old 2.x and 3.x version annotations.
GTK+ 4 is a new start, and from the perspective of a
GTK+ 4 developer all these APIs have been around since
the beginning.
