Added better play/pause handling and cleanup of image loading

[bitboxer]

This PR does three things:

• It adds a README, closing Add README with plugin action summaries #21 and Added README.md #15.
• It cleans up the thumbnail loading and moves the duplicate code to a method
• The important thing: it remembers the last playing item and plays/pauses that one. If more than one item was playing, it will resume all. If you press pause on the Stream Deck and then manually play something, the Stream Deck will notice that and go back to displaying a pause button and let you pause the one item that is currently playing. This mimics now how the play/pause is behaving on macOS with the stock StreamDeck software. This closes Play/Pause should have an option for "last item that was playing" #14.