Add ext to filename when user forget it when rename

This is one solution among two I think.
The second one is change visibility of files the user can choose
from when opening saved workflow.
The first is simpler I guess and it seems more reasonable.

Author: krystofair

orangecanvas/application/canvasmain.py

@@ -1564,6 +1564,8 @@ def save_scheme_to(self, scheme, filename):
             return False
 
         try:
+            # enforcing ".ows" extension during saving file.
+            filename = filename if filename.endswith('.ows') else f"{filename}.ows"
             with open(filename, "wb") as f:
                 f.write(buffer.getvalue())
             self.clear_swp()