introduce new child models and Decoration

[yjbanov]

Add new abstractions:

• Decoration - decorates its child's element
• SingleChildParent - a node with exactly one child
• ElementBase - an HTML element with a flat list child model
• SingleChildElementBase - extends SingleChildParent, an HTML element with a single child model
• LeafElementBase - an HTML element with no children

A couple of applications of the above:

• Container - a decorated box with a single child, uses SingleChildElementBase
• EventRecognizer - captures events of the given type, uses Decoration

[jonahwilliams]

RIP baristaID! I will check it out and then we can discuss something this week.

[yjbanov]

@jonahwilliams let's start combining our work. Merging.