Fix empty quality slider on start.

It seemed that:
- MachineManager was not connected to all the correct signals
- After connecting to the correct signal, ProfilesModel was not yet updated when looping over it in SidebarSimple

Because ProfilesModel's constructor already requests the MachineManager, we cannot do connect itemsChanged the way around as well.

CURA-4707

cura/Settings/ProfilesModel.py

@@ -49,6 +49,10 @@ class ProfilesModel(InstanceContainersModel):
             ProfilesModel.__instance = cls()
         return ProfilesModel.__instance
 
+    @classmethod
+    def hasInstance(cls) -> bool:
+        return ProfilesModel.__instance is not None
+
     __instance = None   # type: "ProfilesModel"
 
     ##  Fetch the list of containers to display.
@@ -91,7 +95,6 @@ class ProfilesModel(InstanceContainersModel):
 
     ##  Re-computes the items in this model, and adds the layer height role.
     def _recomputeItems(self):
-
         # Some globals that we can re-use.
         global_container_stack = Application.getInstance().getGlobalContainerStack()
         if global_container_stack is None: