WebKit thumbnailer

[Jannis Pohlmann]

Hey,

Jerome, I just tested your WebKit thumbnailer plugin and it works for
me. I adjusted the coding style a tiny bit and made the thumbnails be
generated from based on a 1024x1024 frame instead of 1024x600 (since
thumbnails are more or less supposed to have quadratic dimensions). 

I rebased it on master and pushed it to jeromeg/webkit-thumbnailer
instead of webkit-thumbnailer.

Anything else that needs to be done except maybe add a few comments
here and there?

Cheers,
Jannis