fix and feature update for mixins

...not have a value for it, add an override specifier for mixins so they can continue to override all properties on the base (default) or set it to false to defer to any properties already set on the base, add instance and global hasMixin methods to programatically determine if an instance of kind already has the mixin applied to it.

Enyo-DCO-1.1-Signed-off-by: Cole Davis <cole.davis@lge.com>