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It used to be in CppTools, but since the hover handler only makes sense
in the context of the C++ editor, this is a better place.

This change removes a number of wrong usages of the CppModelManager::documents()/document().

Enables F1 for opening their help pages. Seems to work nicely.

The macro tooltip is supposed to be a fallback, since otherwise you
won't be able to see type information for macro parameters.

Reviewed-by: Roberto Raggi