Fix incorrect cover art

[nvllsvm]

• Clear any lingering hints that contain a previous track's cover.
• Load the cover image for all states. Ensures the latest cover is always used.

Resolves https://github.com/eworm-de/mpd-notification/issues/19.

[eworm-de]

Clearing the hint has some side effects, for example it clears urgency and category we set before. I added a first notification without icon in db63b70b5dd04179aabf5119f0d8fc1d51a0edad. Usually this is updated instantly and never seen by the user.

My behaviour regarding when to show the artwork was intended, I did not want to show artwork without track information. However I like the idea to show info for paused state, so changed that in 2885c76fbfc2d2b59994b969761cad8940a0a2f7.

Does this solve all your issues?

[nvllsvm]

All is working and solves my issues. Thanks.