ENH: Don't batch process when show/hide only a few child nodes

When there are many child nodes in a hierarchy then show/hide is much more efficient if batch processing is enabled.
However, if there are few nodes only then a full refresh at the end of a batch processing takes longer than doing the update on each node separately.
Count the number of child nodes and if there are less than 30 then update each node separately.

From: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22662 3bd1e089-480b-0410-8dfb-8563597acbee