Implement named slots

[ChristianUlbrich]

Slots can have names; those need to be reflected to the wrapped element.

For this to work, I think you need to figure out the slots of the wrapped element; that could be done by rendering it invisible / offscreen to get an actual implementation. Maybe we need to monkey-patch all DOM-adding functions, to sniff for <slot> elements...

However if a component is dynamically adding slots, we are out of luck...