Closes #887

The problem was that an empty line was first measured with the editor in a hidden
state, producing dimensions that were all zero. This result was then cached and
reused when the editor was unhidden.

Closes #921

Added "dirty counter" so we can tell when the user has undone/redone to the last point at which the document was marked clean.

Closes #918

Issue #881

Seeds the delta-to-pixel conversion with a value on browsers where
we (possibly) know it. Don't stop the actual wheel event, but let
it scroll normally, but use the estimated scroll delta to proactively
draw the content that will be scrolled into view.

Closes #913

Closes #914
Closes #915

It's no longer allowed on IE10, and not needed on IE9.

Closes #911

Issue #907

(It used to only notice when a single span filled the whole line.)

It's already cached globally, but such cache lookups are not that cheap.

Cut down on round-trips (and improve caching) by measuing all
characters in a line at once, and caching the resulting object, rather
than forcing an (expensive) DOM layout for every char that is
measured, and caching per-char.

That were used to find the precise width at which something wraps.

To allow arbitrary folding of text, and replacement with widgets.

Remove old line folding system.

To prevent repeated re-sorting and general silly side effects.

.. to be a little less gigantic and convoluted.

Prevents weird side effects after setting the selection on an
unfocused editor.