Localization support

[michaelrsweet]

Add support for localization of the web interface using any string resource files that are added.

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Breaking this down into sub-tasks:

• Add pappl_loc_t type and structure to hold per-printer localization information.
• Add cups_array_t *localizations array to pappl_system_t structure to cache the various localizations.
• Add pappl_loc_t *papplSystemFindLoc(pappl_system_t *system, pappl_printer_t *printer, const char *language), papplLocFormatString(pappl_loc_t *loc, char *buffer, size_t bufsize, const char *key, ...), and const char *papplLocGetString(pappl_loc_t *loc, const char *key) APIs, plus corresponding private functions _papplLocCreate and _papplLocDelete.
• Add pappl_loc_t *papplClientGetLoc(pappl_client_t *client) API to return the localization associated with the client's Accept-Language header.
• Update papplClientHTMLPrintf and papplClientHTMLPuts to try localizing their inputs using the (cached) localization info and to set/restore the POSIX locale so that numbers are formatted properly.
• Add base localizations for EFIGS
• Add unit tests

Might be able to internally base the caching on the strings files we are pulling instead of just on the printer since many printer applications won't use per-printer localization files but instead set them at the system level.

[tillkamppeter]

Currently vendor options look ugly:

Option/choice names are the human-readable strings of the PPD file "translated" to IPP keyword syntax. One understands them but real human-readable strings in the web interface would be great.

[michaelrsweet]

All done.