Set machine_extruder_count in sync with defined extruders

The registry checks if they are in sync and complains if they aren't. The extruder count is set back to default to 1 upon adding the machine.

Contributes to issue CURA-3497.

resources/definitions/custom.def.json

@@ -22,5 +22,12 @@
             "7": "custom_extruder_8"
         },
         "first_start_actions": ["MachineSettingsAction"]
+    },
+    "overrides":
+    {
+        "machine_extruder_count":
+        {
+            "default_value": 8
+        }
     }
 }