Cache the thumbnail of Comic Covers and not the full Image

This commit improves the performance by adding the scaled extracted comic cover into the KImageCache instead of the full sized image. Before the full sized image was loaded from the cache and scaled on the fly to the correct size. The size does not change widely from one run to the next, so the cached image will look fine.

This is also very important for the Android version of Peruse as this saves CPU resources which is helpful on a battery powered device.