Restart flanger LFO on Song::playbackStateChanged signal

[SecondFlight]

#4360, but on stable-1.2.

I'm unable to test this, could someone who has 1.2 builds set up test this and make sure it works as advertised?

[zonkmachine]

Tested on stable-1.2, works fine.

NB. The test project in #3689 (flange.mmp.zip) includes noise so the result won't be precisely the same if you don't turn it down.

[zonkmachine]

I will test this some more. Most of the time it will export fine and will give the same result as rendering from the command line. Except for every tenth render which will differ. Investigating.

Edit: I suspect that the glitch seen is due to my testing method

[SecondFlight]

Thanks for testing, @zonkmachine!

I figured out the build process for 1.2-stable and tested this, and it appears to work just fine on my end. The only reason I could see why the 10th render would fail is if the event I'm listening to doesn't fire every 10th render for some reason. But based on your edit I think this is good to merge. I'll do that tomorrow if nobody objects before then.