Add persistent disk cache for library pixmaps

I'll reference #4379

When viewing the library with album covers visible, the covers load very slowly the first time
as they have to wait on the tagreaders. If I scroll down the library, it takes minutes for the
tagreader to catch up. The nice thing is that the pixmaps are cached. However, once
Clementine is restarted, the whole process has to happen again.

This patch adds a persistent disk cache in the form of a QNetworkDiskCache to store the
pixmaps on disk and load them into the QPixmapCache as required.

I've noted literally night and day performance improvements, not only when scrolling through the library.
There is much better interface responsiveness when searching, and I no longer see the no_cover_icon
temporarily anymore.