CURA-4972 Moved validation to SettingOverrideDecorator.py

**The diagnosis:**
The issue arrises from the fact that while the original object is subscribed to be validated, that subscription is only created by setting it as a settings override object. A clone of that object, while still a settings-override object, never gets subscribed because it's not explicitly set with the tool.

**The solution (?):**
I moved all the validation stuff over to SettingOverrideDecorator.py, and use its onSettingChanged() function to trigger the validation. Unfortunately, I can't use the timer because of some limitation with QTTimer and threads. So it's _a bit laggy in some places, and I'd be open to tips about how to fix this. It does work reliably though.

plugins/PerObjectSettingsTool/PerObjectSettingsTool.py

@@ -44,7 +44,6 @@ class PerObjectSettingsTool(Tool):
         self._error_check_timer.setSingleShot(True)
         self._error_check_timer.timeout.connect(self._updateStacksHaveErrors)
 
-
     def event(self, event):
         super().event(event)
         if event.type == Event.MousePressEvent and self._controller.getToolsEnabled():
@@ -156,10 +155,12 @@ class PerObjectSettingsTool(Tool):
 
     def _onPropertyChanged(self, key: str, property_name: str) -> None:
         if property_name == "validationState":
-            self._error_check_timer.start()
+            # self._error_check_timer.start()
+            return
 
     def _updateStacksHaveErrors(self) -> None:
-        self._checkStacksHaveErrors()
+        return
+        # self._checkStacksHaveErrors()
 
 
     def _checkStacksHaveErrors(self):
@@ -180,6 +181,7 @@ class PerObjectSettingsTool(Tool):
 
 
     def _checkStackForErrors(self, stack):
+        print("checking for errors")
         if stack is None:
             return False