Its useful to have an event emitted when all of the sceneQueue tasks
have completed since the metadata can hook this for processing.
Therefore add such an event.
(Bitbake rev: 38d4f65bf1cbcdd5a2d60dff0e1d2859c34ed62e)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>