Make sens.

To avoid introducing a "hard" dependency between the volume rendering module and the editor module, the associated UI element should be enabled only if the volume rendering module is available.

which label map is selected is not explicitly stated in the current UI. Having
a pull down menu with the available choices would make sense. The attached
mock-up is a possible solution to the two stated issues.

This has been addressed in r24326

//------------
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.
//------------