Cache for appOwnershipCached

[Revadike]

Unless I'm missing something, instead of always requesting the product info of all owned games at the start of logging in (when this option is set), we could use cached product info saved on disk and only refresh the ones with an outdated changenumber?

For someone with my library size, waiting for appownershipcache takes several minutes. Not to mention, the large amount of data I have to request from Steam every time.

[Revadike]

Related to #188

There, you worried about picsCache not being accessible synchronously. However, you can keep picsCache in memory, as usual, but also asynchronously save it to file. Only read the cached files into memory upon startup.