support transformers with an unknown number of outputs

To support different types of nodes in the build graph, we introduce
the base class BuildGraphNode. Artifact now derives from BuildGraphNode.
A RuleNode class is introduced that represents a rule in the build graph.
Rules are applied in the build phase and not in a pre-build phase
anymore.
The handling of moc has been revisited. The fixed automoc pre-build
phase is no more.

This is the squashed merge of a feature branch.

Task-number: QBS-370
Change-Id: If27cdc51cba8c9542e4282c2caa456faa723aeff
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>