ENH: allow LabelEffect subclasses to selectively disable some features

Paint Over and Threshold options may not be possible to implement
for some label effects, so make it possible for the subclass to disable
display of the widgets.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21773 3bd1e089-480b-0410-8dfb-8563597acbee