Text tree layout

[rksm]

Towards performance improvements for text layouting by using a btree representation.

[rksm]

This changes

1. Document representation of text (i.e. how lines + strings and attributes in lines are handled)

btree that makes read and modifications efficient.

is represented as

2. What text layouting is based on (i.e. the system for font measurements changed)

document.createRange().getClientRects() if possible, fallback to the slower element.getBoundingClientRect() if necessary.

Generally, concrete measuring is done as little as possible.

3. Interface changes in text morph to make using rich text easier

text.textAndAttributes = [
  "this\n", {fontSize: 14},
  "is a\n", {fontSize: 30},
  "lively\n", {textDecoration: "underline"},
  "text", null
]