Multi-Function Device and Standalone Scanner Support

In order to support multi-function devices and standalone scanners, PAPPL should provide APIs, interfaces, and objects for scanning, with the goal of providing eSCL protocol endpoints (eSCL is used for AirPrint and Mopria) that can be linked to a printer object and/or used separately.

Specific sub-tasks:

• Implement pappl_scanner_t object based on pappl_printer_t (adapted to the needs of scanning - 90% duplication here)
• Update pappl_job_t object to support print or scan jobs (add a pointer to pappl_scanner_t)
• Implement pappl_sc_driver_data_t structure based on pappl_pr_driver_data_t, with corresponding callback functions using the pappl_sc_ prefix. For the purposes of PAPPL scanning, do not support vendor options.
• Implement pappl_sc_options_t structure based on pappl_pr_options_t. For the purposes of PAPPL scanning, do not support vendor options.
• Add scanner.h header with public API and scanner-private.h header with private API using appropriately-named header guard macros. These can be patterned after the corresponding printer headers.
• Add scanner.c file that provides the scanner object constructor and destructor.
• Add scanner-accessors.c file that provides the scanner object accessor (get/set) functions, patterned on printer-accessors.c
• Add scanner-driver.c file that provides the scan driver interfaces and attributes, patterned on printer-driver.c.
• Add scanner-webif.c file that provides scan-specific web pages.
• Add job-scan.c file that provides scan job processing functions.
• Add papplPrinterSetScanner API to set the scanner associated with a printer
• Add papplSystemSetScannerDrivers API to set the list of scanner drivers, callback, etc.
• Add pappl_sc_autod_cb_t, pappl_sc_create_cb_t, pappl_sc_driver_cb_t, and pappl_scanner_cb_t callbacks to system.h.