Move QtConcurrent configuration to a single file

Move QtConcurrent configuration to a single file

This file lives in src/concurrent, alongside the rest of the library. Relevant
configuration was moved out of qglobal.h, as it isn't relevant for other
parties and thus isn't needed there.

This introduces a global header that all QtConcurrent headers now include. This
header includes qglobal.h and defines library-specific configuration for all to
follow.

Change-Id: If6f11e7bbc6139d29004eb1602bd579b75b637c8
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>