UI: Capitalize default filenames from "untitled" to "Untitled"

Capitalize the default filename used for .blend files and other savable
and exportable file formats (like images, 3D formats, etc.) from
"untitled" to "Untitled".

Pull Request: https://projects.blender.org/blender/blender/pulls/132424

scripts/modules/bpy_extras/io_utils.py

@@ -64,7 +64,7 @@ class ExportHelper:
         if not self.filepath:
             blend_filepath = context.blend_data.filepath
             if not blend_filepath:
-                blend_filepath = data_("untitled")
+                blend_filepath = data_("Untitled")
             else:
                 blend_filepath = os.path.splitext(blend_filepath)[0]