BUG: #1696 LabelStatics regenerates plot on each SetParameter call

Added calls to StartModify()/EndModify() to limit the regeneration
of the plot until after the node is fully populated.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23198 3bd1e089-480b-0410-8dfb-8563597acbee