[pwa_extras] Provide default images

Possible child of 🐛 Defined App Image Icon isn't used Fixed

Postponed on 🐛 Defined App Image Icon isn't used Fixed and I think will need changes, once that issue is fixed.

🐛 Defined App Image Icon isn't used Fixed is fixed now, but there isn't much of a relation anyway. There is no 'default_image' (use theme image as app icon) anymore. Instead, if no image is provided by the user, the module uses the provided image icons found in the "assets" folder as a fallback (before these images were always used, 'default_image' is kind of misleading).

Although I doubt these can be used for apple peripherals. As stated here: https://webhint.io/docs/user-guide/hints/hint-apple-touch-icons/ Apple requires very specific image sizes.

So the game plan here should be to add the required apple sizes in the asset's folder as a fallback and apply the image manipulation done for the 192x192 and 144x144 icons inside 'ManifestConfigurationForm' 'submitForm()' for the apple-touch-icons.