Jnygaard jpg proxy

Have tested extensively with the new example jpg_proxy_test only, but it should be backward compatible. Both qt-controller and trell module should work.

New "magic" tinia element names: ap_useJpgProxy (bool) and ap_jpgQuality (int, should be restricted to 0, ..., 100)

Added TRELL_PIXEL_FORMAT_RGB_JPEG_VERSION as a new "pixel_format" for requests of type TRELL_REQUEST_PNG. An alternative (perhaps more logical) would be to add TRELL_REQUEST_JPG as a new request type, and re-use TRELL_PIXEL_FORMAT_RGB. Change this?

(In principle, these could also be combined, e.g., to

  TRELL_REQUEST_RGB_JPG
  TRELL_REQUEST_RGB_PNG
  TRELL_REQUEST_RGB_AND_DEPTH

and so on.)

Have done a few whitespace changes, for increased readability, and tried to do this in separate commits, but see that in the pull-request-diff everything is of course mashed together...