Modified DcmQuantColorHashTable to perform lazy initialization.

To improve performance, DcmQuantColorHashTable now creates the internal
items when they are accessed for the first time instead of at its
initialization.
Thanks to Takeo Satomi <takeo.satomi@gmail.com> for the suggested
improvement.
Fixed typos in comments.
Made sure that the move originator is cleared.

Made sure that the members MoveOriginatorAETitle and MoveOriginatorMsgID are
reset in the clear() method.