Improve layout of HTML animation

[timhoffm]

PR Summary

This closes #11795 and superseeds #11816.

Layout changes:

• Figure is left aligned instead of centered (consistency with static figures).
• Controls are centered below the figure.
• Slider length = (roughly) length of button row - exact value may depend on the browser since the slider width is is still hard-coded and the size of the space between the buttons might vary.
• All buttons have the same size.
• Adjusted vertical spacing between controls.
• Labels are now associated with the radio buttons (you can also click on the label instead of the button for selection).

This is how it looks like: (top: old style, bottom: new style)

[dopplershift]

Thanks for cleaning this up properly with some more detailed CSS. Looks great!

[mgeier]

Thanks a lot, that's great!