Move CuraStage back to where plugins expect it.

plugins/MonitorStage/MonitorStage.py

@@ -2,7 +2,7 @@
 # Cura is released under the terms of the LGPLv3 or higher.
 import os.path
 from UM.Application import Application
-from cura.UI.CuraStage import CuraStage
+from cura.Stages.CuraStage import CuraStage
 
 
 ##  Stage for monitoring a 3D printing while it's printing.

plugins/PrepareStage/PrepareStage.py

@@ -4,7 +4,7 @@
 import os.path
 from UM.Application import Application
 from UM.PluginRegistry import PluginRegistry
-from cura.UI.CuraStage import CuraStage
+from cura.Stages.CuraStage import CuraStage
 
 ##  Stage for preparing model (slicing).
 class PrepareStage(CuraStage):

plugins/PreviewStage/PreviewStage.py

@@ -4,7 +4,7 @@
 import os.path
 
 from UM.Qt.QtApplication import QtApplication
-from cura.UI.CuraStage import CuraStage
+from cura.Stages.CuraStage import CuraStage
 
 from typing import TYPE_CHECKING, Optional