ENH: fixed issue #2771 kill preivious storescp executables

The new logic will get the os process list and if (other) storescps are on
the list, it would warn the user and through a dialog asks for killing
these executables. The start listener button is checkable now and it is only
checked when the listner is running.

From: Alireza Mehrtash <mehrtash@bwh.harvard.edu>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23649 3bd1e089-480b-0410-8dfb-8563597acbee