enhance main.js

Do not initialize the embed module for fluent animations.
Add strict pragma.
Remove unused modules
Next step towards general tab registry.

Add tabs module without registry just helper functions.
Extract chapter, downloads and share tab logic.
This is still in progress for chapters at least and will likely be
refactored further.
Merge branch '2.1.x' of github.com:podlove/podlove-web-player into 2.1.x

Merge changed onPlayerEnded event handler manually.
Removed built production file. Remember to rebuild it yourself with:
`browserify main.js -o static/podlove-web-player.js`

Conflicts:
	js/player.js
	static/podlove-web-player.js