honor "Property.EmitsChangedSignal" annotations
This is just a simple rebase of the patch by @Schnouki from #542 (closed). I've not much use for gdbus-codegen myself, so I haven't tested. Hopefully this will just be easier to review and we can finally get this merged.
I've just done this since it causes a "downstream" bug in audacious that affects my project. I'll ping the author of audacious and ask if they can test this (unless Thomas wants to), and if so I'll remove the WIP tag.
fixes #542 (closed)