Support for Mixins

[trusktr]

Currently, Mixins do not work properly in some cases because the Mixin helper generates new classes, so for example multiple applications of a mixin may generate multiple classes that are all technically from the same definition, but because instances of these classes are from technically different class instances, things like private properties will not work when one instance tries to read private properties from an instance of the supposedly same class definition (because technically it is from another class).