Consider user preference when opening project files from cli.

The logic using just a "skip_project_file_check" boolean was too obscure
imho, so used a string value with more explicit values
to improve readability

CURA-6824

cura/SingleInstance.py

@@ -87,7 +87,7 @@ class SingleInstance:
                 if command == "clear-all":
                     self._application.callLater(lambda: self._application.deleteAll())
 
-                # Command: Load a model file
+                # Command: Load a model or project file
                 elif command == "open":
                     self._application.callLater(lambda f = payload["filePath"]: self._application._openFile(f))