Web: Add dom differ (virtual dom)

[s77rt]

Follow up from #12

Attempts:

1. Used react internal differ by using createElement; this resulted in weird behaviour and in complexity in syncying dom with state
2. Used https://github.com/fiduswriter/diffDOM; encountered two bugs: text nodes are double encoded and newlines does not work well on Safari

I'm thinking of writing a simple dom differ for our use case

[s77rt]

WIP #19