Add encoding='utf-8' for text file reading

CURA-4875

When encoding is not provided, the behaviour is system dependent and it
can break on OS X.

cura/Settings/ContainerManager.py

@@ -486,7 +486,7 @@ class ContainerManager(QObject):
         container = container_type(container_id)
 
         try:
-            with open(file_url, "rt") as f:
+            with open(file_url, "rt", encoding = "utf-8") as f:
                 container.deserialize(f.read())
         except PermissionError:
             return { "status": "error", "message": "Permission denied when trying to read the file"}