Drastically improve the node color change performance
Drastically improve the node color change performance

The volume node is observe by the setupMergeLabel() function that is
called upon the ModifiedEvent. Without the StartModify() and EndModify(),
each time the node was modified the function was called, slowing down the
process a lot.
Now the modified event is only called one at the EndModify().

See Issue #13882

ricortiz/Bender,finetjul/bender,finetjul/bender,ricortiz/Bender,ricortiz/Bender,finetjul/bender,vovythevov/Bender,vovythevov/Bender,finetjul/bender,ricortiz/BenderCraniosynostosis,ricortiz/BenderCraniosynostosis,ricortiz/BenderCraniosynostosis,vovythevov/Bender,ricortiz/Bender,ricortiz/BenderCraniosynostosis,vovythevov/Bender