[MenuBar, Catalyst] Investigate support for loading menu with all available image sources

Catalyst currently only supports Font/File based images and they currently don't use our `IImageSourceLoader` services because the retrieval of the images needs to be synchronous and we don't have synchronous APIs for this.

One option would be to pre-load all images referenced by the xplat `MenuBar` before triggering Catalyst to update the application menu.

Start by tracing the xplat MenuBar image references into Catalyst's application-menu update path, then inspect how IImageSourceLoader services currently retrieve images. Compare the synchronous Font/File handling with the proposed preloading approach. Done means documenting or implementing a clear way for Catalyst to load all supported image sources before updating the menu.

Written by the indexing model from the issue text.