Make sure MachineManager initializes after ContainerRegistry does

CURA-4084

When MachineManager gets created, it will find and set the current
active machine. This requires the ContainerRegistry to be initialized
first.

cura/CuraApplication.py

@@ -119,6 +119,12 @@ class CuraApplication(QtApplication):
 
     Q_ENUMS(ResourceTypes)
 
+    # FIXME: This signal belongs to the MachineManager, but the CuraEngineBackend plugin requires on it.
+    #        Because plugins are initialized before the ContainerRegistry, putting this signal in MachineManager
+    #        will make it initialized before ContainerRegistry does, and it won't find the active machine, thus
+    #        Cura will always show the Add Machine Dialog upon start.
+    stacksValidationFinished = pyqtSignal()  # Emitted whenever a validation is finished
+
     def __init__(self):
         # this list of dir names will be used by UM to detect an old cura directory
         for dir_name in ["extruders", "machine_instances", "materials", "plugins", "quality", "user", "variants"]:

cura/Settings/MachineManager.py

@@ -115,7 +115,6 @@ class MachineManager(QObject):
     activeStackValueChanged = pyqtSignal()  # Emitted whenever a value inside the active stack is changed.
     activeStackValidationChanged = pyqtSignal()  # Emitted whenever a validation inside active container is changed
     stacksValidationChanged = pyqtSignal()  # Emitted whenever a validation is changed
-    stacksValidationFinished = pyqtSignal()  # Emitted whenever a validation is finished
 
     blurSettings = pyqtSignal() # Emitted to force fields in the advanced sidebar to un-focus, so they update properly
 
@@ -306,7 +305,7 @@ class MachineManager(QObject):
         self._stacks_have_errors = self._checkStacksHaveErrors()
         if old_stacks_have_errors != self._stacks_have_errors:
             self.stacksValidationChanged.emit()
-        self.stacksValidationFinished.emit()
+        Application.getInstance().stacksValidationFinished.emit()
 
     def _onActiveExtruderStackChanged(self):
         self.blurSettings.emit()  # Ensure no-one has focus.

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -77,7 +77,7 @@ class CuraEngineBackend(QObject, Backend):
         self._scene.sceneChanged.connect(self._onSceneChanged)
 
         # trigger auto-slicing on error check finished
-        Application.getInstance().getMachineManager().stacksValidationFinished.connect(self._onStackErrorCheckFinished)
+        Application.getInstance().stacksValidationFinished.connect(self._onStackErrorCheckFinished)
 
         # Listeners for receiving messages from the back-end.
         self._message_handlers["cura.proto.Layer"] = self._onLayerMessage