private static String buildSubKey( String key, String subKey ) {
		int dot = key.lastIndexOf( '.' );
		return (dot >= 0)
			? key.substring( 0, dot ) + '.' + subKey + '.' + key.substring( dot + 1 )
			: key;
	}
Inserts {@code subKey} at last dot in {@code key}.
<p>
E.g. {@code buildSubKey( "TitlePane.font", "small" )} returns {@code "TitlePane.small.font"}.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatUIUtils.java