Use Composer autoloading for *all* PHP files

[szepeviktor]

@aristath What do you think about moving all class files into /src and load them with Composer?

Now there are very diverse solutions in this theme.

[szepeviktor]

And clean them by removing all side-effects e.g. new ClassName()

[szepeviktor]

[off] I see you work for ThemeFusion :) Have you heard about https://packagist.org/packages/szepeviktor/composer-theme-fusion and https://packagist.org/packages/szepeviktor/composer-envato ?