Deprecate QDeclarativeDebugHelper class

Deprecate QDeclarativeDebugHelper class

This was a helper class for enable debugging in 4.7. Since Qt 4.8 you
should just say CONFIG += declarative_debug

Change-Id: I51dabaec36e175a097721570061aad38c3f828d1
Reviewed-on: http://codereview.qt.nokia.com/3507
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Aurindam Jana <aurindam.jana@nokia.com>