Fix issues with sequence diagram UR and clipboard

This was regression introduced in the recent UR changes (c4e71ef2).

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

CCBUG: 270217