Fix setting relationships to update UI properly.

Author: Frederic98

cura/BuildVolume.py

@@ -810,6 +810,7 @@ def _updateUsedExtruders(self):
         for extruder in global_container_stack.extruderList:
             used = extruder in used_extruders
             extruder.definitionChanges.setProperty("extruder_used", "value", used)
+        global_container_stack.definitionChanges.setProperty("extruders_used", "value", [extruder.position for extruder in used_extruders])
 
     def _nodeActiveExtruderChanged(self):
         self._updateDisallowedAreasAndRebuild()

resources/definitions/fdmextruder.def.json

@@ -27,6 +27,17 @@
                     "settable_per_meshgroup": false,
                     "type": "extruder"
                 },
+                "extruder_used":
+                {
+                    "default_value": true,
+                    "description": "Is this extruder used for printing. This setting is controlled by Cura automatically.",
+                    "label": "Extruder Used",
+                    "settable_globally": false,
+                    "settable_per_extruder": true,
+                    "settable_per_mesh": false,
+                    "settable_per_meshgroup": false,
+                    "type": "bool"
+                },
                 "extruder_prime_pos_z":
                 {
                     "default_value": 0,

resources/definitions/fdmprinter.def.json

@@ -289,16 +289,16 @@
                     "settable_per_extruder": false,
                     "settable_per_meshgroup": false
                 },
-                "extruder_used":
+                "extruders_used":
                 {
-                    "default_value": true,
-                    "description": "Is this extruder used for printing. This setting is controlled by Cura automatically.",
-                    "label": "Extruder Used",
-                    "settable_globally": false,
-                    "settable_per_extruder": true,
+                    "default_value": [],
+                    "description": "A list of extruder positions that are used. This setting is controlled by Cura automatically.",
+                    "label": "Extruders Used",
+                    "settable_globally": true,
+                    "settable_per_extruder": false,
                     "settable_per_mesh": false,
                     "settable_per_meshgroup": false,
-                    "type": "bool"
+                    "type": "[int]"
                 },
                 "machine_nozzle_tip_outer_diameter":
                 {
@@ -2494,7 +2494,7 @@
                     "enabled": "machine_heated_build_volume",
                     "settable_per_mesh": false,
                     "settable_per_extruder": false,
-                    "depends_on_settings": [ "extruder_used" ]
+                    "depends_on_settings": [ "extruders_used" ]
                 },
                 "material_print_temperature":
                 {
@@ -2608,7 +2608,7 @@
                     "settable_per_mesh": false,
                     "settable_per_extruder": false,
                     "settable_per_meshgroup": false,
-                    "depends_on_settings": [ "extruder_used" ]
+                    "depends_on_settings": [ "extruders_used" ]
                 },
                 "material_bed_temperature_layer_0":
                 {