Let frontend not be a D-Bus service, remove tryPPD(), bug fixes (#29)

- Let the frontend not be a D-Bus service, only the backends
  Controlling of filtering the backend's list of printers to hide remote
  printers and/or temporary queues was done by the frontend being
  a D-Bus service sending signals and not the backends which are
  D-Bus services by themselves just having methods for that. Having
  both frontends and backends being D-Bus services makes the
  interface unnecessarily complicated, especially for distro-independent/
  sandboxed packaging, like snapping. No we use backend methods
  for that and have done away with the frontends being a D-Bus service.

- 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.

- send_printer_state_changed_signal(): Send correct signal
  Send CPDB_SIGNAL_PRINTER_STATE_CHANGED, not
  CPDB_SIGNAL_PRINTER_REMOVED. Thanks to Tim Shimmin from
  Papercut for discovering this.

- Let "dialog_name" always be "const char *", to eliminate warnings.

- Unified logging. Always use log...() functions.

Co-authored-by: Till Kamppeter <[email protected]>