BUG: Fixed strange toolbar popup after DICOM export

After invoking DICOM export from the subject hierarchy context menu and closing the dialog (either by successful export or canceling the window), the Slicer toolbar context menu appeared in place of the subject hierarchy context menu. Its root cause remains to be identified, but the bug was fixed by changing the handling function from mouse release to mouse pressed. Note that mouse pressed was the original behavior, which was changed recently (rev25884).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25898 3bd1e089-480b-0410-8dfb-8563597acbee