xpra is outdated, and uses outdated version of libavcodec

[Øyvind 'bolt' Hvidsten <bolt@dhampir.no>]

xpra in backports is version 0.13.7, while version 0.14.8, bringing 
significan improvements and LTS ( https://www.xpra.org/trac/wiki/News ) 
is available from the winswitch repo's.

Also, both versions use libavcodec-53 from debian, while libavcodec-55 
is available in backports.

Using libavcodec-55 would presumably allow the use of the h264 codec, 
and prevent this error message:
-----------------------------------
buggy avcodec version detected: (53, 35, 0)
cowardly refusing to use it to avoid crashes, set the environment variable:
XPRA_FORCE_AVCODEC=1
to use it anyway, at your own risk
cannot import dec_avcodec (avcodec decoder): unsupported avcodec 
version: (53, 35, 0)
xpra client version 0.13.7
video decoder dec_avcodec could not be loaded: unsupported avcodec 
version: (53, 35, 0)
-----------------------------------

- Øyvind