Container buttons

[dabbler0]

Introduces a new Model entity, buttonContainer, which is invisible in the view except that it has buttons at the end of it. The advantage of requiring a container is that it:

• defines a scope for the button mutation
• provides reparse boundaries, so that the entire block context doesn't need to always be reparsed, and
• it usually has semantic meaning, unlike the old arbitrary-token-in-model approach.