Closes #4548

Add getHelpers and registerGlobalHelper, allow modes to specify arrays
of helper names, support modeProps in mode specifications.

Update foldcode and show-hint to make use of more powerful helper
system by defining an `auto` helper that tries multiple helpers, and
using that as default.

(Old newFoldFunction interface preserved for backwards-compatibility.)

Closes #1133

Closes #1123

The off pun is rather awful, but it does have precedent, and is
more succinct.

This allows multiple handlers to listen for a single event, and makes
the API a bit more uniform.

Issue #725

- Multiple gutters are explicitly created with the gutters
  option
- Markers go into a specific gutter, new setGutterMarker,
  clearGutter methods
- The line number gutter is separate
- The 'fixedGutter' feature is removed (painful to do now,
  was shaky anyway)

Courtesy of Daniel Glazman

Since v2 is the canonical CodeMirror at this point

A folded block will now automatically unfold when the line containing the unfold
widget is deleted.

So that I can put other doc-related stuff there without polluting the
root dir.

It's tiny anyway, no need to force people to include yet another file.

Much nicer than onKeyEvent

The latter strips newlines in IE9