Move metadata to build flavor specific location

I believe a Weblate wizard is needed to finish this.

New setup:

• in /metadata Full and Basic share some metadata:
  • changelogs (and translations)
  • screenshots (these can be deleted and flavor specific ones added to /src)
  • feature graphic (same here, though there's probably no need)
• in /src they've got flavor specific metadata:
  • full description (and translations)
  • short description (and translations)
  • title (and translations)
  • icon

Closes #2893 (closed)