Mention CodeMirror UI and add a note about line wrapping.

CodeMirror no longer has a concept of 'character width', since that
was just too broken -- tabs and non-ascii characters can have non-
standard widths, even in monospace fonts. We now build up a DOM
structure and measure its width whenever we have to determine the
width of something.

The textarea would still have a 'reduced' selection, so replacing the selection
by typing or pasting would not replace anything.

When both horiz and vertical scrollbars were present, clicking the horiz
one would be interpreted as a click in the code, and start a drag
event.

This is a provisionary change for now. I'll finalize it and document it
when it has been proven to be useful.

This way, scrolling won't cause the horizontal scrollbar to jitter,
and the line PRE elements can be given a background that spans the
whole editor.

This reverts commit 0b4b4a81.

Firefox raises an error when reading selectionStart on a textarea
that's not in the DOM.

Before, if you had an onCursorActivity callback that performed
any modifying operation on the editor, it would prevent onChange
from being fired

when a css rule assigning width: 100%, border&padding: 0 existed
for textareas, the editor would slow down to a crawl on webkit
for some unfathomable reason

this is actually much nicer-looking (don't need to scroll it
into view) and safe (since you can't scroll the editor
without unfocusing the dropdown)

i forgot to rename a use of a variable when i renamed its def