ITargetFactory: Make supportedTargetIds() and canCreate() orthogonal

ITargetFactory::supportedTargetIds should return all targets that this
factory can support, regardless of e.g. installed qt versions.

Whereas canCreate() should check the availability of e.g. a qt version.
This makes supportedTargetIds() have the same meaning as
ITargetFactory::supportsTargetId().

Change-Id: I744abb89e441026d6f3f8de21944d780e9b9fa32
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>