Improve caching

[pabloyoyoista]

Currently, there is some small caching working when fetching updated apps. However, the caching could be extended to avoid other costly calls, most remarkably refines. For example, whenever a user opens to see the description of an app, it gets at least 2 to 3 refines for the same app. The caching could be then invalidated whenever the repositories are refreshed. There is a risk that GS gets out-of-sync with APK if people use both GS and the terminal. But I guess the target should be rather non-experienced users than power-users.