ENH: Added show option to Add data dialog for volume loading
'Show' is enabled by default and so volume that is loaded is displayed. However, user can disable 'Show' option and then current view settings will be preserved.
Also removed switching to batch processing mode during volume loading, as this is a very expensive operation (takes time, screen flickers, etc) and it is not necessary anymore.
The option is also accessible from Python:
slicer.util.loadVolume('fixed.nrrd',{'show':False})