Allows navigating the code using the mouse when pressing Ctrl. Suggested
a few times and also seen in other IDEs.

Task:     240811

Fixes: Add a combo for diff file browsing, bring diff editors to front, exclude qconfig.pri from files examined by p4.

Task: 234842

Many singletons have their own instance() method, in which case there is
no need to use ICore::instance() to get access to them.

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

forward declaration is sufficient.

RevBy: Friedemann Kleint

    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.