First layout fix of tabs

- Delete all gradients and other unused styles from tabs elements
- Replace style 'height:auto' for a fixed height in tab.js
- Comment-out hard-coded css-styles for class 'tab' in chapter.js
- Wrap anchor-elements in controlbar with ul and li elements
- First attempt to create mixin / classes for css-arrow as indicator for state active in tabs
- General make-up of tabs elements according to new layout
Merge branch 'tabs' of https://github.com/podlove/podlove-web-player into tabs

Conflicts:
	js/modules/chapter.js