I18n: fix UI layout operator context extraction

Some operator layout buttons with custom text did not get the proper
context: they'd get * instead of Operator.

This was probably never noticed because the only operator that
actually had this issue was Import Images as Planes in the Add menu.

Reviewed By: mont29

Differential Revision: https://developer.blender.org/D15993

release/scripts/modules/bl_i18n_utils/bl_extract_messages.py

@@ -570,7 +570,7 @@ def dump_py_messages_from_files(msgs, reports, files, settings):
         bag = extract_strings_split(node)
         opname, _ = bag[0]
         if not opname:
-            return i18n_contexts.default
+            return i18n_contexts.operator_default
         op = bpy.ops
         for n in opname.split('.'):
             op = getattr(op, n)