Closes active window, if active window is not main window for a monitor.
Calling CSV_MM_WinFree rather than WinFree verifies that assigned monitors
maintain at least one open window. That window will be the one
opened by the CSV_MM_PageDisplay
function.
Always call CSV_MM_WinFree to close a window if
multi-monitor functionality has been implemented.
Return Value
0 if successful, otherwise an error is returned. -1 indicates that you attempted to close the main window of a monitor.