Move livepreview debugservice into qtdeclarative

The preview service debug service is the enabler on the Qt side for QML livepreview. It is only a tiny piece of software, yet we spend a disproportionally big amount of time discussing and wondering how to get it into release packages or device images. In fact, we don't have a clear plan yet on how to release it to customers with custom Qt versions.

Let's simply add this little module to proper QtDeclarative and be done with it. 5.12 is a good target.

The debug service, being a development tool, is not distributed and used on end systems, so it does not generate potential runtime business. Therefore, the default QtDeclarative license should be just fine. However, in order to satisfy the company-internal request to use GPL/Commercial as a license, this particular debug service may have be released under that license.