Add cups interface for printing

See https://snapcraft.io/docs/cups-interface

Closes #11

snap/snapcraft.yaml

@@ -25,6 +25,7 @@ grade: devel
 confinement: strict
 compression: lzo
 license: LGPL-2.0-or-later
+assumes: [snapd2.55] # for cups interface
 
 layout:
   /usr/share/libdrm/amdgpu.ids:
@@ -51,6 +52,15 @@ layout:
     symlink: $SNAP/usr/bin/dot
   /etc/povray:
     symlink: $SNAP/etc/povray
+
+# this is not used or needed for anything other than to trigger automatic
+# installation of the cups snap via "default-provider: cups"
+plugs:
+  foo-install-cups:
+    interface: content
+    content: foo
+    default-provider: cups
+    target: $SNAP_DATA/foo
 
 environment:
   LD_LIBRARY_PATH: $SNAP/usr/lib/$SNAPCRAFT_ARCH_TRIPLET/blas:$SNAP/usr/lib/$SNAPCRAFT_ARCH_TRIPLET/lapack # numpy
@@ -82,6 +92,7 @@ apps:
       - network      
       - browser-support
       - unity7
+      - cups
   cmd:
     command: usr/bin/FreeCADCmd
     extensions: [kde-neon]