Qbs: Allow setting of properties

This enables support for e.g. setting CFLAGS, custom defines, etc.

Task-number: QTCREATORBUG-9380
Change-Id: I9d4a560ac5ba8a40c51c15fdf8009c0dba5eeef1
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Doc: remove an obsolete step in using Qbs

The executable is now selected automatically in the run
settings when using Qbs.

Change-Id: I528e0a7e067349f9925e13b927ca402c3543a46d
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>