Qbs: Support subprojects

Change-Id: Ic7015b44362b89a14957f7d21540e71d441c860f
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Qbs: Use different Icons for products and groups

With subprojects the tree is getting too confusing when everything
uses the same icons.

Change-Id: Ib1b8559c5f6e8a3c4299498db67d33c9eb916d66
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>