Sort the scripts and scenes into folders

main/ holds the main window and top bar, quiz/ the quiz and its parts,
player/ the music player, popups/ every popup and what they share, data/ the
songs, players, playlist and settings, server/ the MusicServer client,
common/ the list row and GameLog, and themes/ the themes and style boxes.

RELEASING.md

@@ -5,7 +5,7 @@ Releases are made from the version in `project.godot`
 same version in the About window.
 
 1. Bump the version, e.g. from `1.9.0` to `1.10.0`, and add a `WHATS_NEW` entry in
-   `about_popup.gd`.
+   `popups/about_popup.gd`.
 2. Merge to `main` and push.
 
 The `Build` workflow (`.gitea/workflows/gitea-build.yaml`) then: