Closes #641

Removes use of getElementsByClassName

And fix test suite which was demanding the wrong result.

Closes #410

Closes #342

Closes #307

This is quicker to update, and allows querying both by height and by
line number, which will help with the text-wrapping and code-folding
implementations.

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.