add 'transform to platform newline' function to the GDK String API

Triple quoted strings are very useful in unit tests. But there is a problem when you create the expected value with triple quoted strings and the actual value is for example produced by a writer. On windows the two values are not equal, as triple quoted strings use always '\' as the line separator. I don't know the reason why triple quoted strings behave like. But it would be nice to have a straight forward way to turn a triple quoted String into its platform specific form. The PlatformLineWriter is offering such a functionality, but it is obviously not that straightforward to use it. Therefore I propose to have a new method for the GDK String API that does what PlatformLineWriter does: gobbles any \r characters and replaces every \n with a platform specific newline.