Switch from imageio to pyav

[tfaehse]

• Native ffmpeg audio encoding in one go, no more subprocess calls
• Much better performance!
• Add video stabilization option
• Aborting from the UI actually works reliably now, without any thread kill hacks

Roadblocks:

• Some audio formats can't be copied over to the output video, maybe resample them?
• Give the UI a more logical layout

[tfaehse]

imageio-ffmpeg has shown to be (at least) as fast, so there's no need for this anymore.