Added move layer up and down

- simplified the way a layer is deleted. Now a single strech is added at the
end of the layers. So no extra random streches are found lost between the layers.
- also the ui file associated with the editor has been added. Forgot to add
it earlier. Most probably the previous two commits won't compile without it.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>