Metaclass to construct CustomViewer and subclasses
The metaclass does two things when constructing new classes:
it finds the class-level attributes that describe ui elements (eg checked=False). It bundles these into a ui dict attribute, later used to construct the FormElements and widgets to represent each setting
It creates the qt DataViewer widget class associated with this class.
It looks for overridden user-defined methods like plot_subset, and registers them for later use.