Protect our users from metadata spam in streams.
Some streams resend their metadata every few seconds, so everytime the OSD comes up, which is quite annoying.
This should fix those streams in all cases.

Streams trying to spread advertising messages between normal metadata will only be able to show each message exactly one time, as long as the distance between those is not larger than 12 metadata changes.

This only protects the OSD, statusbar and current track applet are not affected, as here those messages are by far less annoying.

I hope App.cpp is the correct place for this, as the OSD gets shown here.