cpdb-backend-cups 2.0b6

In the sixth beta release for cpdb-backend-cups 2.0.0 we have done more changes for sandboxed packaging, job data is streamed through domain sockets now and the frontend not being a D-Bus service any more. And many smaller improvements and fixes.

• Stream print data through a Unix domain socket
  To ease making a Snap from the CPDB backend for CUPS we now transfer the print job data from the dialog to the backend via a Unix domain socket and not by dropping the data into a file (PR #28).
• Let the frontend not be a D-Bus service, only the backends
  To control hiding temporary or remote printers in the backend's printer list we have added methods to the D-Bus service provided by the backends now. Before, the frontends were also D-Bus services just to send signals to the backends for controling the filtering (PR #29).
• Unified logging. Always use log...() functions (PR #29).
• Updated names of some CUPS constants to CUPS 2.5.x and newer (PR #28).
• Removed the backend functions for get-all-jobs(), get_active_jobs_count(), and cancel_job() (PR #28).
• Removed tryPPD(), a useless, PPD-related function
  This function logs the options in the PPD of the CUPS queue but does nothing else. PPDs will also go away in CUPS 3.x, so we want the CUPS backend not depend on PPDs or PPD-supporting functions/libraries (PR #29).
• send_printer_state_changed_signal(): Send correct signal
  Send CPDB_SIGNAL_PRINTER_STATE_CHANGED, not
  CPDB_SIGNAL_PRINTER_REMOVED (PR #29).
• Let dialog_name always be const char *, to eliminate warnings (PR #29).
• Added NULL checks for the functions dealing with dialogs
• Build test: Give more time (3 instead of 1 sec) for the print job submission before closing the backend, to get note of the confirmation of successful submission (PR #28).
• Build test: Let CPDB frontend and backend log in debug mode (PR #28).
• Build test: Create the directory for the socket files (PR #28).
• Updated build test for removal of backend info file