Type VNode and VDomModifier

[cornerman]

We could make VNode generic on its actual dom type. So div could be of type dom.html.Div and so forth. That would mean we have HtmlVNode[E <: dom.html.Element] and SvgNode[E <: dom.svg.Element]. Then we could have an apply method which takes a VDomModifier[dom.html.Element] respectively VDomModifier[dom.svg.Element], thereby we could only apply html attrs to html tags and the same for svg.

Furthermore, we could try to make our EmitterBuilder[dom.Event, R] work with this information. it would be great to have a typesafe way to access the current element from a dom event or hook. Then you could have input( onDomMount.foreach { elem: dom.html.Input => ??? }, onInput.map(_.currentTarget.value) --> sink).

We have helpers like onInput.value as of now but they are not typesafe but just cast to an input element and assumes the value property exists (see #93). We want the compiler to tell us about these problems.

PR #237 makes a first step.