Get rid of private Qt headers in qmljsdebuggerlib

The qmljsdebugger library right now depends on three private Qt headers:

• qdeclarativedebugservice_p.h
• qdeclarativeengine_p.h (to get hold of the script Engine)
• qabstractanimation_p.h (to get hold of QUnifiedTimer)

As we plan to compile in the qmljsdebugger library into the customers executable having these dependencies on private & unstable API is not acceptable.