Replace Split.js with GoldenLayout in demos

[AlexAltea]

GoldenLayout seems to be exactly what we need to make a decent IDE.

[carlsmith]

I haven't used it, but it does look ideal, and was apparently used for the Monaca Cloud IDE (which I haven't used either, but looks the part).

ACE is (these days) very simple to customize and integrate into other projects, and provides a solid text editor out of the box.