Allow multiple instances of the editor to be created in a single page

Mangle HTML ids in a predictable fashion in dialog widgets to ensure they
do not clash if multiple instances are loaded.
Split screen editing app

Creates two editor instances within a single web-page, and allows live
collaboration between them.

Key features:
* EASY TO USE (requires no additional setup to run)
* supports client disconnection
* simplistic latency simulation
Merge pull request #656 from peitschie/split-screen-editor

Split screen editing demo