Simplify low-level DOM API

[mitar]

Current API with folding and aleady* flags is cumbersome. It is questionable how much composability it really offers. Maybe it would be better to simply allow each mixin to guide calling through getFirstMixin and getNextMixin (see #3). And maybe those methods should simply check the status of the DOM before doing anything. If they observe that DOM was already inserted, they do not do anything, for example. Or do it differently.

So in some way communication between mixins would work through node objects themselves. Similarly to how we communicate between event handlers by calling methods on the event object itself.

[mitar]

Fixed with c2ad456.

[mitar]

Simplified API so that mixins are just called in order.