Help requests must be done in main thread.

Help requests must be done in main thread.

Fixes an occassional assert "Cannot send events to objects owned by a
different thread." with receiver QHelpEngineCorePrivate.

Change-Id: I77ffd663a055c493607907374f7a4d775f6aac6c
Reviewed-on: http://codereview.qt.nokia.com/2289
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Christian Kandeler <christian.kandeler@nokia.com>