This causes the main window to signal the application to exit and delete
its own pointer on close.  This fixes an issue where apparently some
windows that aren't explicitly connected to the main window would be
left open when the main window was closed because by default Qt will
only exit when all windows have been closed.

Because it deletes its own pointer, instead of storing it in a
std::unique_ptr, use a QPointer because it has an internal mechanism for
automatically tracking QObject deletion even if the deletion was not
done on the QPointer itself, where as unique_ptr does not have that
functionality.  In other words, if the pointer is deleted elsewhere for
whatever reason, the QPointer will still set that internal pointer value
to null.

(message and minor modificiations by Jim)