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.

project.godot

@@ -23,7 +23,7 @@ config/icon="res://icon.svg"
 [autoload]
 
 GameLog="*res://game_log.gd"
-Settings="*res://Settings.gd"
+Settings="*res://settings.gd"
 Playlist="*res://playlist.gd"
 MusicServerClient="*res://music_server_client.gd"