Name popups *Popup and give scripts, scenes and classes matching names

Every popup is now called *Popup instead of *Window in its file, root node,
node name, class and variable, and PopupWindow becomes BasePopup. Player is
PlayerRow, SongListObject is SongRow and Character is CharacterCard. Script
files are snake_case and scene files PascalCase, each matching its class or
root node, and LightGrayTheme.tres is light_gray_theme.tres.

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
-   `AboutWindow.gd`.
+   `about_popup.gd`.
 2. Merge to `main` and push.
 
 The `Build` workflow (`.gitea/workflows/gitea-build.yaml`) then: