As spotted by the extended linter.

Apparently Opera Win *doesn't* handle them properly, whereas Opera
Linux does.

Closes #929

Just like Opera, it flips the ctrlKey and metaKey on Mac

Closes #924

Closes #926

Issue #923

So that passing in a bogus position doesn't cause a run-time error
and/or corruption.

Issue #928

Remove unused code, fix selection resetting

Things that happen in a single operation are now always combined into
an undo history event. Subsequent typing or deleting is combined, if
it is close together in time, even when done in separate operations.

Closes #889

Also stores old selection position in undo history, restoring it
when applying a changeset.

Issue #923

Holding a key would queue up a screen and cursor update (which requires a line measure)
for each character typed. That could cause the display to lag behind the input in a
visible, obvious way.

This still doesn't make responses instant when editing long lines in Opera, but at least
makes the maximum delay bounded to the time a single update takes.

To not require tons of empty DIVs when a section is hidden.

Issue #920

So that one can change the font size, call refresh, and
not have old cached measurements cause artifacts.

Closes #922

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.