Added support for Parameter ui type hint "presets".

This can be used to use the PresetsOnlyParameterValueWidgets for parameters which have presets but for which presetsOnly != True. This necessitated changing the precedence of type hints within ParameterValueWidget.create(). Matching type hints for all subclasses of the parameter are now considered to be a better match than a more specific parameter type registrations lacking the type hint.
Merge pull request #518 from johnhaddon/presetsOnlyTypeHint

Added support for Parameter ui type hint "presets".