Brr: Better ux for inserting reactive elements in the DOM

[panglesd]

Yo! I finally worked on this.

The way elements are inserted in the DOM in the brr-lwd examples is not ideal. It requires you to understand more of Lwd's API than necessary, as well as request_animation_frame. Moreover it would not work as is for actually reactive elements (only for reactive children, which is the case in the examples).

So this PR introduces two ways to insert reactive elements in the DOM directly into the brr-lwd API. The examples are updated to use the two ways. EDIT: based on what I use on slipshow, I added append_child and prepend_child.

The API mimicks two standard way of inserting nodes in the DOM, insert_siblings and set_children, but with reactive siblings/children.

The main difference is that insert_siblings is insert_sibling: not plural, it takes a single element instead of a t col.
It is not really possible to have an insert_siblings `Replace el seq, as if we replace el by a reactive sequence that is first empty (so we just remove el) and then non-empty, we lost our anchor to know where to add the new elements.

We could still add the following API:

val insert_siblings : [ `Before | `After ] -> t -> t col -> root

and I suggest to do so but in another PR, as it requires to refactor update_children quite a bit.

[panglesd]

Well there is a bug here. Update_children expects no children at the beginning, so it will add them (again) and we end up with a duplicated initial value.
The fix is probably:

Suggested change

	let root = Lwd.observe children in
	let c = Lwd.quick_sample root in
	El.set_children el (Lwd_seq.to_list c);
	Lwd.quick_release root;
	update_children el children
	update_children el children