qml: content image adjustments

Reading from disk is slow, so loading content images block the GUI thread long enough that it causes visible stuttering.

This merge request makes content images loaded asynchronously in a low priority thread, so that the GUI thread is not blocked. This approach fixes the stuttering when (fast) scrolling the playlist, as well as in other situations.

I set a maximum size for the player image. It should make more sense to scale the cover to the maximum size possible, rather than loading it into memory fully.

Player image caching is disabled because in most cases it is too large to be useful for caching. DragItem image caching is disabled to not exhaust the global cache.