Fix new setStyle methods.
Make CodeArea grow with the window in the demo.
Implement scrollToVisible by digging into VirtualFlow.
Improve implementation of PageUp/PageDown.
Up/Down navigation maintains pixel offset.

Previously, character offset was maintained. As we are moving to
officially support non-fixed width fonts, it was necessary move.
Replace StyledTextAreaHelper by interfaces with default methods.
Model text changes as an observable stream of change objects.
Generic UndoManager.
Implement line wrapping, by "brute force".

By brute force I mean that when the width of the text area changes,
all visible lines are replaced (by themselves), which means whole
new TextFlows are constructed and styled for the whole viewport.
No top/bottom padding in line cells.
Override LineCell.computePrefHeight to get the line height right.
Implement hit and navigation for wrapped lines.
Set TextFlow's prefWidth to 0 to fit to ListView's width.
Generic inline-style text area.
Remove unused method.
Expose UndoManager.
Proliferate the use of TwoLevelNavigator.
Renaming and more proliferation of 2-level navig.
Add Position.toString() method.
Don't need to keep track of visible cells anymore.