Infrastructure for supporting multiple profilers in qqmlprofilerservice

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>
V4 profiler adapter

This adapter attaches the V4 profiler to the QML profiling
infrastructure.

Change-Id: Ifaf77a3ba0790bc56d4b155260b0e05593ac799e
Reviewed-by: Kai Koehne <kai.koehne@digia.com>