QbsProjectManager: Improve run config creation.

The current code is overly simplistic, permanently flooding the "run
configurations" combo box with irrelevant entries that make it close to
unusable for even medium-sized projects.
With this patch, the irrelevant products are removed from the list as
soon as we know that they do not produce executables.

Change-Id: I606e7d17fd9eb89c8c0d91a7f41a9092193e175b
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>