The concepts of "global" and "engine" profilers are introduced. Profilers
can either be attached to specific QML engines and run in their threads
or they can be unspecific global profilers running in arbitrary threads.

Change-Id: I3862fc65c07ccb33a1ca08cd2425e4079d3ffc02
Reviewed-by: Kai Koehne <kai.koehne@digia.com>