Update screenshot, add release notes for 0.3.15

I attempted to backport !19 (merged) to the Flathub version but was stymied by the lack of <releases> in the appdata (which is now mandatory). There are several other nice changes that are unreleased (translation updates, new icons, new menu layout) – it would be nice to release them.

This MR updates the screenshot referenced in the appdata, adds <releases> with a description of what will become 0.3.15, and adds the same notes to NEWS. It is less verbose than what is normally in NEWS (roughly speaking, a Git changelog), and it assumes today's date as the release date. (Whomever makes the release can change this, of course!)