Fix issue #258 - bug moving a cel from a transparent layer to background

The problem was in DocumentApi::moveCel(). We cannot move a Cel between
layers and frames at the same time easily (it's not possible to do it
without being in a temporal invalid state, e.g. where two cels are in the
same frame position). It's better if we create completely new cel for
the target and remove the previous one in all cases (undoers are already
prepared to do those operations correctly).

- Add Sprite::getImageRefs() member function.
Add DocumentApi::setCelOpacity()
Timeline: Fix problem clicking first and second frame
Merge remote-tracking branch 'origin/dev' into dev