Scrolling lags due to loading of cover images

[flackbash]

When the base directory contains several albums, scrolling through them is often lagging because the cover image bitmaps for each album need to be loaded into the image view. Implementing a cache in c15cf59 helps when scolling up and down several times within a short period of time such that the images can be retrieved from the cache. However, when scrolling for the first time upon starting the app, scrolling still lags. The solution to this is probably loading images asynchronously.