Cache responses from the AppStore server

Otherwise every time the AppStore was opened a lot of connections to the AppStore server were made which resulted in a terrible performance.

This changeset will cache the response for a sensible time so that only the first request will be somewhat slow.

Performance changes:
- Loading a category took previously more than 3 seconds on my machine. Now for every follow-up request it takes less than 200ms, resulting in a performance gain of 1950%
- Loading the category list took previously about 750ms - now it takes 154ms, a total performance gain of 395%

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