Remove arguments from the constructor.
For actions, we now default to COPY, which is the most common one
that we should enable by default (MOVE requires handling deletion
on the the source side, and ASK only makes sense if we have
multiple actions).
For the content provider, we add a new ::prepare signal where
it should be provided just-in-time.
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.
