Resource adjustments:
 * Include fonts locally (/src/fonts/) rather than via google-fonts.
 * Remove markovDictionaryBuilder from index.html (it's imported by worker script, not needed here)
 * Remove markovDictionaryWorker from index.html (it isn't changing much anymore, so the zealous caching is now beneficial)
Change 'constants' of element selectors into an object (uiElements).
Move "setCurrentStep()" and related functions into new module (UI.workflow)
Large refactor of markov.js, and DOM modifications to account for the huge simplification of processes resulting in that refactor:

* remove buttons
* improve separation between logic and presentation on range inputs
* tidy up functions
* improve markovSourceOptions and markovOutputOptions
* remove the concept of (workflow) steps
* shuffle things around in the UI (and simplify the whole #!)
Remove obsolete markovSourceOptions.sourceText
Prevent the markovDictionaryWorker from building up a large queue of work by terminating and reseting if another 'buildDictionary' is requested while one is currently running.