Allow NEXT widgets to be extended with a custom behaviour

[nextinterfaces]

NEXT widgets should allow a custom code to extend the default functionality.

Dustin: "I was wondering generally if you might open up a lot more classes for extension via protected members and fields as things are in progress?

As one example, I'm trying to use XSwitch and it doesn't appear to expose a basic setValue(boolean) so it's hard to programmatically set the value after creation, and with XTextField I'd like to adjust the 40/60 split of label and widget as needed. All the private members are making this a lot of code copying at this point, and I'd rather address them with local extended classes and then converge back on the standard impl over time if possible. Making all of the base inner widgets and helper methods that configure them (e.g. XSwitch._switch() and _switched() protected would help greatly. Thanks for your consideration!"