What's New/Changelog dialog

### Problem

- App updates are often now automatic which means folks aren't regularly exposed to changelogs.
- When folks do get changelogs, they are between the currently installed version and the update, not between the last used version and the update
- When they get an app redesign they may not know it and be lost or confused when a feature they use has changed.
- When an app gets a new feature, they aren't onboarded to the new feature

### Proposal

Some kind of "What's New" dialog that automatically pops up when an app is updated to a new version. We could repurpose Metainfo changelogs to show new features and bug fixes. It should keep track of the last opened version and show all changes since the last time the app was opened

It needs to support these tags if we use metainfo: https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-description

### Prior Art (Optional)

https://github.com/SvenTiigi/WhatsNewKit