Remove View::minimumSizeHint()

It's a QWidget specific concept, so move it to the qtwidgets
frontendcode. QtQuick won't need it.

src/qtwidgets/views/ViewWrapper_qtwidgets.h

@@ -60,6 +60,7 @@ public:
     Controllers::DropArea *asDropArea() const override;
     bool testAttribute(Qt::WidgetAttribute) const override;
     void setCursor(Qt::CursorShape) override;
+    QSize minSize() const override;
 
     QWidget *widget() const;