News for 0.3.16 release

On Flathub I got a pull request to update the runtime. As a matter of principle I don't like to push updates which only update the runtime, but I see that there are many nice fixes and improvements on master.

I prepared the NEWS and appdata updates for a 0.3.16 release. I assume no offence will be taken if I just make the release myself but I'll wait a few days in case I'm wrong!