This normalizes the classes used by most of the modes
(reStructuredText and diff didn't fit in), and makes them all
reference a central style sheet (under theme/). It also adds a theme
option that can be set to change the theme, and defines a few
different themes (some based on old mode colors).

Doing shift-left-arrow a few times and then shift-up a few times,
you'd get weird jumping selections as you went up.

(Which would make horizonal scrollbars 'stick' after the wide lines had
been deleted.)

See comment in code for details. Fixes #90

This remove the ugly white square popping up right before the
context menu opens on right-click.

This prevents the 'flicker' of black text when typing in coloured
elements.

It would raise an error on a hidden editor because showingTo was 0,
causing no HTML to be produced, although further code assumed there
would be at least one number.

Fixes #110

It causes random cursor movement and text deletion for some
reason.

Fixes #105

When the selection included the end of a line, and you typed a
character, it'd get the offsets wrong and duplicate the part of
the line before the selection.

Fixes #111

This to solve issues with 'CSS reset' packages that gave PRE nodes
a padding.

This might break some client code.

There is now a CodeMirror-scroll class that is used to provide the
scrolling, rather than using the outer wrapper for this. This allows
adding fat borders to the wrapper without causing glitches in
cursor and mouse-click positioning.

Empty or almost-empty lines would cause the highlighter to stop
highlighting, because they didn't get new styles, even when the
styling of the lines around them was completely changed (for
example when adding a script tag around something in HTML mixed
mode)

readInput would, in some cases, take effect even though our textarea
was in context-menu-hack mode.

This might very well break something else, but passes my minimal tests.

So that languages in which blank lines are significant can detect
them.

Apparently, popping up the context menu took more than 50 \(\!\)
milliseconds on that platform, and codemirror had already hidden
the textarea again by the time the menu arrived.