Allow programmer to enabe/disable qml debugging functionality in the users program

Right now the only way to 'disable' the debugging functionality of QtDeclarative is through a configure switch. Anyhow, developers might want to disable it consciously in the first place.

Proposed solution:

• disable declarative debugging by default for release builds
• Allow developers explicit control by a static 'QDeclarativeEngine::setDebuggable(bool debuggable)'