Add the discovered printer functionality

This can be used in the add machine wizard to short circuit the creation of a printer.
So instead of first selecting a printer and then connecting it, it will be possible to have
a list that contains all network discovered printers (from all plugins that can do this).

This means we can select one of those printers and directly add it (thus no longer needing that step)

CURA-6179

cura/CuraApplication.py

@@ -745,6 +745,8 @@ class CuraApplication(QtApplication):
         # Initialize Cura API
         self._cura_API.initialize()
 
+        self._output_device_manager.start()
+
         # Detect in which mode to run and execute that mode
         if self._is_headless:
             self.runWithoutGUI()