[Slayer] Replace all text handling with QString

The runtime uses a multitude of different string types, unifying all those to use QString (or QByteArray?) would make the code a lot more straight forward and make internationalization support easier to implement.

Note: The string table is used for translating between the multitude of string types and translation to unicode, but the string table is also used for other functionality in the runtime. Perhaps best approach is to first add QString support to the string table and then hunt down and change all other string types and convert those to QString.