[webcomponents] Finalize the slots proposal (#128)

The proposal is here: https://github.com/w3c/webcomponents/blob/gh-pages/proposals/Slots-Proposal.md

Task list:
- [ ] Review *slotting algorithm*
- [ ] Review *get distributed nodes algorithm* and rename it to *get composed nodes algorithm*
- [ ] Enumerate all DOM operations that will trigger the *slotting algorithm*. This will be defined as a synchronous action.


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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/128