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).

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.

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)