win32 print system notifications

[Antoine Martin]

The cups backend does give us print job status notification, which we use to delete the print job's temporary file.
Apparently, we should be able to achieve something similar for win32 using ctypes: ​Received Print Job Python

[Antoine Martin]

r16881 exposes the print jobid, we could then use ​catch printer event in python and wait for the DELETED notification for this job before returning from the "print" tool we exec (with a long timeout), then we can just delete the temporary file.

[Antoine Martin]

Done in r17277 (and found some minor bugs - fixes to backport).

Some important notes:

• we wait for the "DELETING" or "OFFLINE" job status, as "DELETED" does not seem to ever get triggered
• the code ends up being more complicated because we have to watch for the job status before we start submitting it, otherwise it may complete so quickly that we never see it and end up waiting forever
• the print job timeout is now configurable: XPRA_PRINT_JOB_TIMEOUT defaults to 3600 seconds (1 hour)

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/1115