Otherwise the compositor gets all confused when it's trying to make
drag happen but we know it's not going to happen.
After all, we exchange data behind its back, we just need to keep it
informed.
We were always adding ASK to the list of possible
actions, and preferring it. This was causing the
ask cursor to show up when both the source and
the target support ASK, even though it is only
meant to happen if you hold the Alt modifier.
Instead, use one of the supported actions as
preferred action.
GLib master propagates argument types in g_clear_pointer(), which causes
the usual function pointer casts to GDestroyNotify to trip compiler
warnings. Additionally, this commit changes some cleanup functions where
appropriate (wl_data_source_destroy ->
gtk_primary_selection_source_destroy for struct
gtk_primary_selection_source).
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.
