Switch QML profiler off when deleting QQmlProfilerService

Like this we don't have to check for QQmlProfilerService::instance
anymore in order to find out if we can use the profiler service. If
instance == 0 then enabled == false. Furthermore if it was thread safe
before it must still be thread safe like this.

Task-number: QTBUG-35315
Change-Id: Ie5737e3586f2431afbf11423ba53fd13dd371feb
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Make sure QQmlVmeProfiler methods can be inlined

Move the definitions to the header and adapt the documentation.

Task-number: QTBUG-35315
Change-Id: I511a192cfe1e3e09ade035175cb8f3f332022143
Reviewed-by: Kai Koehne <kai.koehne@digia.com>