Constraint when media notification is present/absent (aka media is present but might not be playing)

[CamperSamu]

It's basically #598, so it might be added as a config option to "media is playing"

The use case for such a constraint is when remapping an hardware action key on a phone.
In my specific case I want my action case to behave like a media key when a media notification is present (eg. YouTube or Spotify paused in the background) and I want it to behave like a Go to last app button when no media is playing.
Right now if I constraint Go to the last app to media playing it will trigger when I have paused media in the background, the same is true for the play/pause mode since it will never trigger if I constraint it on media not playing.

Developer TODO (don't remove)

• add Media Notification Present constraint
• add `Media Notification Absent| constraint
• update documentation