Synced editor scrolling PoC.

Proof of concept for the synchronised source/preview scrolling in composer (see here: https://meta.discourse.org/t/syncing-the-editor-viewport-scroll/13249).

This PR is probably not suitable for production - code is not fully optimised and there may be some not properly tested edge cases - but I'd like to hear your opinion if the approach I've used works ok before putting more time into polishing the code.