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.

music_list_panel.gd

@@ -5,7 +5,7 @@ extends PanelContainer
 ## Emitted when the row of song number [param song_no] is clicked.
 signal song_clicked(song_no: int)
 
-var _song_list_object: PackedScene = preload("res://song_list_object.tscn")
+var _song_list_object: PackedScene = preload("res://SongRow.tscn")
 
 @onready
 var _rows: VBoxContainer = $ScrollContainer/MusicList
@@ -15,7 +15,7 @@ var _rows: VBoxContainer = $ScrollContainer/MusicList
 func refresh() -> void:
 	Settings.delete_children(_rows)
 	for s: SongObject in Playlist.get_playlist():
-		var new_song_list_object: SongListObject = _song_list_object.instantiate()
+		var new_song_list_object: SongRow = _song_list_object.instantiate()
 		new_song_list_object.song_object = s
 		new_song_list_object.gui_input.connect(_on_row_input.bind(s.song_number))
 		_rows.add_child(new_song_list_object)