I18n: extract UILayout headings with context

Some UI elements (row and column) can define a heading text.
Optionally, this text can use a custom translation context specified
through `heading_text`. This changes allows extraction of those
headings using the appropriate context instead of the default one.

Pull Request: https://projects.blender.org/blender/blender/pulls/122326

scripts/modules/bl_i18n_utils/bl_extract_messages.py

@@ -590,7 +590,7 @@ def dump_py_messages_from_files(msgs, reports, files, settings):
         "msgid": ((("msgctxt",), _ctxt_to_ctxt),
                   ),
         "message": (),
-        "heading": (),
+        "heading": ((("heading_ctxt",), _ctxt_to_ctxt),),
         "placeholder": ((("text_ctxt",), _ctxt_to_ctxt),),
     }