Introduce main window affinity

By default a dock widget can dock into any main window.
With affinities, we can now have a dock widget "belong" to a main window
and only be able to dock into it (or into other floating dock widgets
with the same affinity).

See DockWidgetBase::setAffinity() and MainWindowBase::setAffinity().

src/private/DropArea_p.h

@@ -68,13 +68,18 @@ public:
 
     bool checkSanity(MultiSplitterLayout::AnchorSanityOption o = MultiSplitterLayout::AnchorSanity_All);
     bool contains(DockWidgetBase *) const;
+
+    QString affinityName() const;
 private:
     Q_DISABLE_COPY(DropArea)
     friend class Frame;
     friend class TestDocks;
     friend class DropIndicatorOverlayInterface;
     friend class AnimatedIndicators;
+    template <typename T>
+    bool validateAffinity(T *) const;
     bool m_inDestructor = false;
+    QString m_affinityName;
     DropIndicatorOverlayInterface *m_dropIndicatorOverlay = nullptr;
 };
 }