BUG: Editor: Ensure label map is updated each time a master volume is selected

Before the commit, the following was failing:
(1) Go to SampleData -> Load MRHead
(2) Go to Editor -> Create GenericAnatomyColors label map
(3) Go to SampleData -> Load CTACardio
(4) Go to Editor -> Select CTACardio master volume -> Create GenericAnatomyColors label map
(5) Select MRHead master volume, select CTACardio master volume
-> Popup message with "Warning: Geometry of master and merge volumes do not match."
-> Label map associated with CTACardio is not selected

Note also that this problem is independent of commit r24202 (BUG: Pattern
matching for label volume does not account for underscore)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24207 3bd1e089-480b-0410-8dfb-8563597acbee