replace ExtensionSystem::PluginManager::instance()->getObject<Core::ICore>() by Core::ICore::instance()

forward declaration is sufficient.

Details:  - Conform to conventions.

Introduced pp-scanner.cpp, renamed `pp' to `Preprocessor' and removed useless #includes.

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().

    enum ExtraSelectionKind {
        CurrentLineSelection,
        ParenthesesMatchingSelection,
        CodeWarningsSelection,
        CodeSemanticsSelection,
        OtherSelection,
        NExtraSelectionKinds
    };
    void setExtraSelections(ExtraSelectionKind kind, const QList<QTextEdit::ExtraSelection> &selections);
    QList<QTextEdit::ExtraSelection> extraSelections(ExtraSelectionKind kind) const;

This is mainly for the benefit of the cppmodelmanager.