[API] added SubProgressMonitor class to core

This class should replace the SubMonitor usage.

Change-Id: Ieda8c54f463346278bc4df9c31566eb3c0eec68e
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1824
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>