Asynchronously request the drag operation's contents converted
to the given @type.
When the operation is finished @callback will be called. You must
then call [method@Gdk.Drop.read_value_finish] to get the resulting
GValue.
For local drag'n'drop operations that are available in the given
GType, the value will be copied directly. Otherwise, GDK will
try to use [func@Gdk.content_deserialize_async] to convert the data.
Asynchronously request the drag operation's contents converted to the given @type.
When the operation is finished @callback will be called. You must then call [method@Gdk.Drop.read_value_finish] to get the resulting GValue.
For local drag'n'drop operations that are available in the given GType, the value will be copied directly. Otherwise, GDK will try to use [func@Gdk.content_deserialize_async] to convert the data.