Revert "qml: don't cache large medialibrary covers"

This reverts commit 35210a54.

We can not afford reading images from disk again and again, or afford re-creating expensive images such as the ones in the Genres view.

It is not clear why this was merged, because in principle this (caching images) is what we have been doing since the very beginning. Delegate image that are loaded in one size should be cached.

This fixes twitching when the views are re-opened, or when the views are scrolled.

Request review @chub.