ColorSwatchMetadataWidget : Fix update bug
ColorSwatchMetadataWidget : Fix update bug

This has been broken since ec69691a87f37427d4c9e4a2a5b720c422885b79, always showing white, regardless of the actual value. Also removed `self.__value` because it is now just duplicate state; at one point it was meaningful to have a value of `None`, but that was rather specific to the UIEditor, doesn't make sense for a generic widget, and was also broken by ec69691a87f37427d4c9e4a2a5b720c422885b79.

Fixes #3384.

hradec/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,appleseedhq/gaffer,johnhaddon/gaffer,boberfly/gaffer,lucienfostier/gaffer,johnhaddon/gaffer,johnhaddon/gaffer,hradec/gaffer,GafferHQ/gaffer,boberfly/gaffer,ImageEngine/gaffer,ImageEngine/gaffer,hradec/gaffer,andrewkaufman/gaffer,lucienfostier/gaffer,boberfly/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,ImageEngine/gaffer,johnhaddon/gaffer,lucienfostier/gaffer,hradec/gaffer,hradec/gaffer,GafferHQ/gaffer,andrewkaufman/gaffer,ImageEngine/gaffer,boberfly/gaffer,andrewkaufman/gaffer,andrewkaufman/gaffer,johnhaddon/gaffer