Implemented a Multiple-Selection capability inside the DICOM Browser dialog box.
Note:  Selection in this dialog is done by checking the items, not just highlighting them.
CTRL and SHIFT buttons are working.  Items could selected by holding {CTRL,SHIFT} be either
a) clicked by mouse :  CTRL=flip check state,  SHIFT = check state on
b) using Up\Down keyboard key  : SHIFT = check state on.  CTRL is not allowed.