BUG: fix loading of multiple referenced loadables

Load DICOM with referenced loadables popup always used the first checkbox value
and ignored checkboxes for the remaining loadables.

• in case there were three referenced loadables and second and third were unchecked,
  all loadables were still loaded because only the value of the first checkbox was used
• loadableCnt never got incremented while checking checkboxes

From: Christian Herz <christianherz1985@googlemail.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25908 3bd1e089-480b-0410-8dfb-8563597acbee