Draft: Improve UI responsiveness
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.