This is to go along with the newly introduced GdkDrop.
This commit includes the necessary updates to the X11, Wayland
and Broadway backends. Other backends have to be updated separately.
This uses the new method without GDK_ACTION_ASK:
Either it is a single action (queryable via gdk_drag_action_is_unique())
or it is not and then the drop target has to make a decision
(potentially by asking someone).
The ultimate goal of this patch series is to split GdkDragContext into
GdkDrop + GdkDrag classes for the destination and source side of a dnd
operation.
The refactoring is meant to work something like this:
1. Introduce GdkDrop as a base class
2. Make all drop related code (like GdkEvent) use GdkDrop instead of
GdkDragContext. Move/duplicate APIs to allow that.
3. Port all drop contexts in the backends from GdkDragContext to GdkDrop
4. Delete all APIs in GdkDragContext that aren't needed anymore.
5. Make GdkDragContext no longer a GdkDrop subclass
6. Rename GdkDragContext to GdkDrag
