ENH: Editor-helperBox: Simplify UI using node selector for merge volume

This commit uses a node selector to (1) display the current merge volume
name, (2) allow the user to select a different one and (3) create a new
one if needed.

By using a node selector, it simplifies the code by:

• removing the need for the "mergeName" HelperBox ivar
• removing the need for the "merge" HelperBox ivar and instead add a property
• removing the obsolete LabelSelectDialog dialog

Note that a custom "Create new " action is associated.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24326 3bd1e089-480b-0410-8dfb-8563597acbee