Add playback speed control

[TheMinefighter]

Sometimes you want to watch a talk a bit faster, and for such cases, I suggest the introduction of playback speed controls as already present on media.ccc.de.

[NiciDieNase]

Thanks for the suggestion.
Right now I'm working on other things, but in the meanwhile, if you are missing any features in the integrated player you can already use any player you link.
On phones and tablets, just select 'Play in external player' from the details page of a talk or enable the option to always use an external player in settings.
On TV with the latest version, you can also enable set option in settings.