The plugin framework should avoid the use of the same object in different score

Currently it's possible to get a Chord on a score and add it to another one with the plugin framework.
Obviously, closing one of the score will free all the objects added to a score, and in the other one the objects are orphans and the user experiments crashes.
The plugin framework should clone the added object if they don't belong to the score they are added.

See : http://musescore.org/en/node/6435