Added monitor to check if Spotify is running/playing music.

[calledude]

• Added semaphore in SpotifyVolumeController to make volume-changes more consistent, previously other threads could read the volume-values while they were in the process of being changed, causing unwanted behaviour.
• SpotifyClient now waits longer before checking again if it didn't detect any music playing.
• SpotifyMonitor added. - This class checks if Spotify is running & playing music periodically and updates SpotifyVolumeController and VolumeGuard if the status changed via an event. This will allow the user to change the system-volume whenever Spotify isn't playing music.
• Checking if music is playing instead of checking for an active device, this makes sure that music is actually playing. A device can be active and not be playing music, this fixes that.
• Spinning up a separate thread for the MediaKeyListener since the Open.WinKeyboardHook library refuses to run properly without Application.Run() present.
• Other general code improvements.