Fix issues with sequence diagram UR and clipboard

This was regression introduced in the recent UR changes (c4e71ef).

Summary of fixed issues:
* crash when pasting sequence diagram
* crash and other misbehaviour on UR message/object widget actions
* pasted widgets were added to the document twice (duplicate widget elements
in saved XMI) - happened with all diagram types
* extra: undo removing an object widget did not restore the connected messages
* precondition widgets were never included when pasting a sequence diagram