MPRIS behavior is inconsistent

[haruInDisguise]

Is your feature request related to a problem? Please describe.
The mpris "status reports" emitted by ncspot are a bit inconsistent and sometimes redundant.
For example, 'Metadata' and 'PlaybackStatus' reports are one and the same and both being emitted
upon any change to the player's internal state.
The expected behavior would be a clear separation of those events ('Metadata' being sent upon song changes, etc.).

While the current implementation may technically function for most software, it's lack of clearly separate events might prevent software to work properly with ncspot.

Describe the solution you'd like
I've opened a PR that addresses these issues, in a way that I considered to be "expected"

Describe alternatives you've considered

Additional context
See my PR for additional notes.