QtGraphs pollutes the global namespace

E.g. qtgraphs/src/graphs3d/engine/barinstancing_p.h defines BarItemHolder and exported BarInstancing in the global namespace, outside QT_NAMESPACE.

Acceptance Criteria:

• check that symbols of the library (exported or not) are inside QT_NAMESPACE
• fix those that aren't
• pick back to all active branches, if possible