and make sure there are not too many of these threads
running at any time. This stops the massive process
startup when loading a project with many UI files, etc.

Task-number: QTCREATORBUG-15795
Change-Id: Icfcddd80d04e36b61ecafbbefe5a1a8b7ea02ec6
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>
Reviewed-by: Ulf Hermann <ulf.hermann@theqtcompany.com>