support different backend than snabbdom

[cornerman]

We have a VTree consisting of multiple VDomModifier, this is currently translated into a snabbdom proxy, on which we call patch whenever an observable triggers.

We could make outwatch independent of the backend. So you can have our modifier definitions and handling of observables, but can chose between different backends. One backend would be snabbdom, it takes the modifiers, transforms them into VNodeProxy and sends them to snabbdom. A different backend could work without a VDom and just change the dom on each update.

This would require us to abstract over the snabbdom events like onInsert, onDestroy, etc. They do not do what the user expects anyway, so I think we can refactor this part of the API.