[documentation] fixed missing inherited properties for `me.ColorLayer` and `me.ImageLayer`

also removed the overwritten update function, since we do not change
anything compared to the original `me.Renderable` one.
[documentation] fixed given `me.event.subscribe` KEYDOWN and KEYUP examples
small update to the trimming functions and added a corresponding test unit.
Merged the drag'n'drop unit test into the "official one"

Not really sure what I did is 100% right, but so far it seems to be
working ;p