Honour QSizePolicy::Fixed too

Some widgets, for example QPushButton, instead of having an explicit max size,
they communicate the need for a max size through their size policy

src/private/multisplitter/Widget.h

@@ -55,13 +55,14 @@ public:
     explicit Widget(QObject *thisObj)
         : m_thisObj(thisObj) {}
 
-    virtual ~Widget() {}
+    virtual ~Widget();
 
     virtual void setLayoutItem(Item *) = 0;
 
     // Not strickly necessary, but it's nice conveniance for kddw which is widget based.
     virtual QWidget *asQWidget() const { return nullptr; };
 
+    virtual QSize sizeHint() const { return {}; }
     virtual QSize minSize() const = 0;
     virtual QSize maxSizeHint() const = 0;
     virtual QRect geometry() const = 0;
@@ -102,6 +103,9 @@ public:
         return obj == m_thisObj;
     }
 
+protected:
+    static QSize boundedMaxSize(QSize min, QSize max);
+
 private:
     QObject *const m_thisObj;
     Q_DISABLE_COPY(Widget)