Make ModelWidget opaque
Vars allows chaining by returning instance from setter
Most Widget commands return the Widget (not getters) to allow chaining
Derived Widget.layout()s return self
Using Lua colon syntax for instance function definitions
Typo
Add Widget.add_to_root() shortcut method
Register widget callbacks with event bindings, not in ctor
Replace add_to_root() with more generic add_to()
Use new API features in derived Widgets