Add order-only dependencies

Qbs operates on artifact level in general. But sometimes it is necessary to enforce waiting for all artifacts of another product. Example:

• project contains autotest-runner
• autotests need non-executable artifacts from product A and B when running
• there is no rule that specifies a dependency on the artifacts and it would be clumsy to explicitly write a dummy rule that does nothing.

Suggestion:

• Add property waitForAllArtifacts or something similar to Depends item