refactor player module

Move non-player dependencies to app module.
Move (nearly) all rendering from player to app module.
Player.create expects a callback to modify the player after it was
initialised / replaced.
Merge branch 'tabs' of github.com:podlove/podlove-web-player into tabs

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