Draft: Improve UI responsiveness

Open Henrik Tunedal requested to merge tunedal/fdroidclassic:lagfix into main

This addresses issue #43.

The main cause of the UI lag is that there's a lot of I/O happening on the main thread, mostly through the ContentProvider classes and PackageManager. Much of it is triggered for each list item as it's scrolled into view.

I have moved some of that I/O work onto background threads, which seems to improve things substantially. I also added a cache for the icon URL and version info, which is cleared in onStop and by the "Update Repos" button.

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