FlatpakReferencesModel: Set fallback app icon if none was found in bundle
This happened to Konsole, due to a mix of packaging error and flatpak-builder refusing to copy required more generically named icon from base platform package. Apps should provide their icons, of course, but we should also display something sensible if they don't.
@apol said that "application-vnd.flatpak.ref" is not an appropriate icon to use for this case, but I think it at least looks less boring than "package-x-generic" which is also applicable to Flatpak runtimes and SDK packages, and packaging sources in Discover.
Edited by ivan tkachenko