ENH: Allow in-place conversion of scalar volume to labelmap

If the combobox selecting the target labelmap is set to 'Replace current node', which is the None selection, then the currently selected node will be deleted, and the new node will be selected in Volumes module, so it will seem that only the type of the current node was changed.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24918 3bd1e089-480b-0410-8dfb-8563597acbee