printing enhancements: cups backend status

[totaam]

To populate more fully the printer information in some print dialogs (see screenshot), we would need to add support for the side-channel and/or back-channel:

• Filter and Backend Programming
• backend

Sadly, this is not exposed in pycups.. so we would need to use ctypes.

[totaam]

2016-06-13 07:11:34: antoine uploaded file print-dialog.png (55.0 KiB)

a print dialog with an xpra forwarded printer

[totaam]

See also #1344

[totaam]

The commit above should show something in the printer status, at least briefly.
To make the backend more useful, we need to change the print backend to get feedback from the server and potentially also from the remote client(s) - which is much more difficult.
At the moment we just execute a send-file subcommand.