Add an identifier string to all Qt binaries to identify that Qt is being used

The use case is that it should be possible for publishers (e.g. Ovi) to detect whether the application uses Qt, so that the right dependencies can be included in the download.

The easiest thing would be to have a string that occurs in all binaries, which can then be "grepped" for. The string should be detectable inside the application binary both when linking dynamically and statically.