Misc topology issues in qan::Graph

[cneben]

• When dragging a node, save it's z, drag with graph maxZ then restore z position in endDragMove().
• collectSubNodes() -> rename to collectOutNodes() / collectChilds() (more consistent with other collectChilds method).
• All collect methods returns containers of const qan::Node* which is particularly painfull.
• collectInnerEdges() should be named collectAdjacentEdges().
• Add a "generic version" of all theses methods with support for std::vector / std::unordered_set by default. Add support for sketchy Qt types (QList, QVector).