Video Support including Fading

[tobi08151405]

Extension of Basic Video Support

This PR builds upon #333 .

This PR provides:

• Fade of video cues using the "Alpha" plugin
• System sink expanded to also use autovideosink
• Automatic adjustment and linking in the pipeline for audio-/video- and audiovideo-cues

TODO

• [x] Fade of Audio and Video currently happen after eachother not concurrently
• [x] adjust other video parameters (brightness, contrast, etc)
• [ ] most of the TODOs from #333

[tobi08151405]

@FrancescoCeruti I could not figure out how to let the fade of audio and video happen simultaneously when fading a Media with Audio an Video. I marked the functions as async, but that did not help. (See lisp/cues/media_cue.py)

[FrancescoCeruti]

@tobi08151405 since Fader.fade() is blocking, you cannot simply iterate over them, or they will run sequentially. async is a whole different world.

What I would do is one of these two:

• refactor BaseFader and subclasses to allow for multiple "targets" at the same time
• create a FadersGroup class that can "orchestrate" multiple Fader objects, it should implement a fade() method that, using a ThreadPoolExecutor, can start the faders and wait for them to finish

[tobi08151405]

@tobi08151405 since Fader.fade() is blocking, you cannot simply iterate over them, or they will run sequentially. async is a whole different world.

What I would do is one of these two:

* refactor `BaseFader` and subclasses to allow for multiple "targets" at the same time

* create a `FadersGroup` class that can "orchestrate" multiple `Fader` objects, it should implement a fade() method that, using a `ThreadPoolExecutor`, can start the faders and wait for them to finish

I implemented a FaderGroup as suggested, I also refactored the whole alpha/fade pipeline which can be handled much easier

[tobi08151405]

I now added a few basic video filters. And I also fixed up the review notes of #333 , without the separate pipelines, since I removed them in my branch

[linuxnow]

Could this be included in next version? This is a game changer :)