refactor(kff.base, kff.View): move DATA_ constants from kff.View into kff namespace
feat(kff.Binder): add a new binder modifier :evf (eventFilter) to process DOM events through a function

The filter function must accept two arguments: 'fn' - the original event handler and 'event' - the original DOM event. When the event is accepted, the function must call the original handler: fn.call(this, event).