See discussion in https://github.com/marijnh/CodeMirror/pull/984

Closes #995

Useful for add-ons that need to register keys.

array initialization (one entry per line) results in extra indentation from second element onwards:
ex:-
const string[] fields = {
  "one",
    "two",
    "three"
};

Issue #986

Issue #986

This stores the previous value for a mode property if an extension
overwrites it with a new value. The old value is stored in a property
prefixed with an underscore (e.g. "_token"). This allows extensions to
piggyback the old value by still being able to access it.

This allows the xml mode to return a style with space separated classes
and still work. I didn't use /\btag\b/ since that can match "tag-foo".

Closes #1002

Apparently, IE does a redraw as soon as you assign to scrollTop/scrollLeft.

Closes #992

Issue #990

As a die-hard vim addict, long ago, I retrained myself to type Ctrl-[ to
leave insert mode. This sends the same control sequence as escape in
many other places, actually, and this patch brings this functionality to
CodeMirror.

From vim's :help ctrl-[

                                        *i_CTRL-[* *i_<Esc>*
<Esc> or CTRL-[
    End insert or Replace mode, go back to Normal mode.
    Finish abbreviation.
    Note: If your <Esc> key is hard to hit on your keyboard, train
    yourself to use CTRL-[.

Typing CTRL-[ is particularly easy when caps-lock is remapped
to being another CTRL key, since now neither hand needs to move away
form the homerow, and both little fingers just slide a little, the left
little finger slides to the right to where caps lock used to be, and the
right little finger slides a little bit up to the '[' key

The 'inverted' hack which hailed from the ancient times when selection
was handled by the hidden textarea, has been replaced with a saner explicit
anchor and head. setSelection now directly sets the anchor and head, rather
than always creating a forward selection.

Even when they move by only one pixel. This prevents the gutter from
coming to rest in a one-pixel-off position.

That was a bad idea.