Preprocess metadata cache if non-existing (issue #2246)

[edwood-grant]

Short summary

This aims to fix #2246

Forcefully deleting the cache will create an odd situation where the app loads the main banner and disappears after a few seconds (probably when the caches have been loaded in the worker thread), leaving the main window in an odd incomplete state.

This change looks up in the FlatpakBackend.vala constructor for the user and system cache metadata folders and will recreate them (via preprocess_metadata) if they are not found.

This will incur in a load time penalty. I believe this is not much of a big deal since this only happen if you explicitly delete the cache folders, which should be a rare occurrence in general.

Tests

Same procedure as set in the bug:

• Kill App Center
• Remove in ~/.cache the folders io.elementary.appcenter, flatpak and appstream (you can also simply delete the ~/.cache folder itself.
• Open App Center

This fix should show the banner and recently updated icons without issue.

Additional info

I believe this is probably the least expensive and more complete solution. There might be a case for just letting the main banner stand and do nothing... but I'm not sure about that. You'd expect all the times to have a fully loaded presentation in the main window, no matter if it's the first time or if you have cleaned your cache.