Change animation controls API

[elenatorro]

Improve animations-controls-widget API

• [x] Use a variable from the viz style
• [x] Show progress value
• [x] Add examples
• [x] Add documentation in README.md
• [x] Add bridge documentation in README.md

[Jesus89]

I found a little issue because the slider overflows in the animation control:

This can be easily solved by removing overflow: auto from the as-animation-controls-widget class.

[Jesus89]

There is another mini-issue. When the header is empty it occupies a blank space:

[Jesus89]

The as-widget-header adds still a blank space when there is no title/description