Add "stage-packages: poppler-utils" to cups-filters part.
Adding "stage-packages: poppler-utils" makes the snap include the
utilities needed by some Poppler-based filters as there is no access
to the system under "strict" confinement.
"Classic" confinement is not supported in Ubuntu Core and also not in
many non-ubuntu distros, so let us go with "strict" confinement to
make one universally usable printing stack snap.
For the purpose of using the snap as a base for printer driver
plugin snaps we want to install the snap on a classic system, so
we continue develeopment for making it work on a classic system
now.
Removed use of MuPDF and removed use of the daemon-dbus plug
For now we are switching over to Poppler as the PDF interpreter as we
want to use Poopler anyway, because Poppler is needed to support
printing of test pages and flattening filled PDF forms.
Classic systems do not provide a daemon-dbus slot, so we have to
remove the daemon-dbus plug to use the snap on classic systems.