Make libQmlDebug usable for QtCreator

Most of the code in libQt5QmlDebug is duplicated in Qt Creator. This is a rather painful state of affairs. The implementations constantly get out of sync and some features are added to one but not the other. libQt5QmlDebug should just expose a public API and QtCreator should use it.

A precondition is that the different client implementations are fully synchronized. We want, for example, fully featured debugger and inspector clients there.

Such an undertaking would require a migration strategy, where we keep a sufficiently compatible qmldebug library in QtCreator until we can really rely on the one in Qt to fulfill all our needs.