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/integration/test_stats_after_win.gd

@@ -12,7 +12,7 @@ func before_test() -> void:
 
 
 func _stats_checkbox() -> CheckButton:
-	return _quiz.get_node("SettingsWindow").get_node("%AddToDatabaseCheckButton")
+	return _quiz.get_node("SettingsPopup").get_node("%AddToDatabaseCheckButton")
 
 
 func test_winning_turns_statistics_off() -> void:
@@ -57,7 +57,7 @@ func test_songs_after_a_win_are_not_counted() -> void:
 
 func test_winning_score_cannot_go_below_one() -> void:
 	_press("QuizToolbar/SettingsButton")
-	var lower_button: Button = _quiz.get_node("SettingsWindow").get_node("%LowerButton")
+	var lower_button: Button = _quiz.get_node("SettingsPopup").get_node("%LowerButton")
 
 	for i: int in 3:
 		lower_button.pressed.emit()