cups: add comprehensive CUPs setting page

- Add printers
- Delete printers
- Use polkit APIs as fallback on auth errors
- Fix ref system to conditionally subscribe to cups when wanted

core/internal/server/cups/testing.go

@@ -0,0 +1,13 @@
+package cups
+
+func NewTestManager(client CUPSClientInterface, pkHelper PkHelper) *Manager {
+	return &Manager{
+		client:   client,
+		pkHelper: pkHelper,
+		state: &CUPSState{
+			Printers: make(map[string]*Printer),
+		},
+		stopChan: make(chan struct{}),
+		dirty:    make(chan struct{}, 1),
+	}
+}