And manually compensate for our own scrollbar sizes.

This way, we're no longer dependent on the size of our scrollbars
being the same as the size of native scrollbars, and can start to
think about custom scrollbars.

As long as CodeMirror.normalizeKeyMap is called on them before they are
used. This interface is a little weird, but backwards-compatible.

And adjust vim mode to use these.

If "{{" was found, the code would highlight the rest of the line
regardless if a matching "}}" or "}}}" was found.

Also update the mode loading demo to make use of the info in that
file, allowing filenames and mimes to be entered.

Issue #2828

Closes #2623

Issue #2529

Issue #2494

It has been superceded by keymap/sublime.js.