Make sure Braille Printer Application is completely independent of CUPS 2.x and of libppd, and that it is snappable

[tillkamppeter]

The Braille Printer Application should not just find the classic Braille printer driver in a local CUPS installation, finding that installation using CUPS methods. Otherwise we could just install the classic Braille printer driver and use the Legacy Printer Application of pappl-retrofit to make the driver available in the new format. We need a native Printer Application which also works on systems without CUPS 2.x and especially on immutable distros or IoT/embedded.

• It should be independent of the used CUPS version, of the CUPS daemon, and of the CUPS directories.
• It can use the CUPS library, but it must especially work with libcups3 (libcups of CUPS 3.x) and optionally with libcups2 (of CUPS 2.x).
• It should not use PPD files or *.drv files.
• It should not use libppd.
• To get the functionality of filters which come with CUPS or with cups-filters the filter functions of libcupsfilters (2.x) have to be used.
• This repo should not install a classic CUPS driver any more, the Printer Application also works with CUPS 2.x.
• Filter executables of this repo which have still the format of classic CUPS filters should not be installed in CUPS' directories, to avoid clutter, but in the Printer Application's own directories, like /usr/lib/braille-printer-app/.
• The whole thing should be easy to package as Snap with a snapcraft.yaml similar to the ones of the hp-printer-app or of LPrint.

[tillkamppeter]

@arunpatwa ^^