Integrate CodeMirror v4 and implement multiple selections

Landing the CodeMirror v4 work into master. See the [multiple selections spec](https://github.com/adobe/brackets/wiki/Research:-Multiple-cursors-and-selections) and the [CodeMirror v4 upgrade guide](http://codemirror.net/4/doc/upgrade_v4.html) for more details.

Thanks to @marijnh for all his work on multiple selections in CodeMirror v4.