This adds custom proxy file storage to the sequencer.
Reasoning: sometimes low resolution versions are already available as a
seperate file built by the capture application.
So there is no real reason to make blender build it's own seperate proxy.
This also somewhat fixes (aehm works around :) )
[#13632] Creating Proxy Takes Over Process, Should be Background
Since now you can just fire up ffmpeg to build your proxies in the background.
(You could have done before and build a directory of jpeg files, but then you
would have to rename all the files since otherwise things are off by one...)
ea94f8ab8a