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/MainWindowBase.h

@@ -100,6 +100,29 @@ public:
     ///@brief returns the MultiSplitterLayout.
     MultiSplitterLayout* multiSplitterLayout() const;
 
+    /**
+     * @brief Sets the affinity name. Dock widgets can only dock into main windows of the same affinity.
+     *
+     * By default the affinity is empty and a dock widget can dock into any main window. Usually you
+     * won't ever need to call this function, unless you have requirements where certain dock widgets
+     * can only dock into certain main windows. @sa DockWidgetBase::setAffinityName().
+     *
+     * Note: Call this function right after creating your main window, before docking any dock widgets
+     * into a main window and before restoring any layout.
+     *
+     * Note: Currently you can only call this function once, to keep the code simple and avoid
+     * edge cases. This will only be changed if a good use case comes up that requires changing
+     * affinities multiple times.
+     *
+     * @p name The affinity name.
+     */
+    void setAffinityName(const QString &name);
+
+    /**
+     * @brief Returns the affinity name. Empty by default.
+     */
+    QString affinityName() const;
+
 protected:
     void setUniqueName(const QString &uniqueName);