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.

tests/fakes/main_window_test_suite.gd

@@ -84,10 +84,10 @@ func _add_player(player_name: String) -> PlayerObject:
 
 
 ## Returns the row in the player list that shows [param player_object].
-func _player_row(player_object: PlayerObject) -> Player:
+func _player_row(player_object: PlayerObject) -> PlayerRow:
 	var rows: Node = _quiz.get_node(
 			"PlayersPanel/VBoxContainer/ScrollContainer/PlayersListVBoxContainer")
-	for row: Player in rows.get_children():
+	for row: PlayerRow in rows.get_children():
 		if row.player_object == player_object:
 			return row
 	return null