Allow offline support through a caching mechanism of the Steam games lists

[AmineI]

By caching the Steam API calls & responses, and using cached responses on network errors, this enables offline support if a folder was already opened online once before.

The cached responses are used instead of the network calls, until they expire (3 days by default). unless there was a network error, in which case we use the cached responses regardless of their expiration.

This uses the module requests-cache

Along with the art lists caching mechanism (which caches the urls of available arts), Kodi's own cache (which caches the arts and items themselves), this should be good to go for full offline support.

[AmineI]

I tried to make this PR stand alone (by not depending on #20 in case it's not merged), so it will likely have merge conflicts if #20 is merged - due to :

• settings.xml's "delete-cache" which I added to both
• addon.xml's dependency on requests-cache which I added to both
• main.py's "delete_cache()" which was defined in both

I'll monitor that and fix conflicts accordingly in case you want approve both PRs Conflicts resolved, ready to merge !