Add experimental support for minimization to side-bar

Auto-hide not working yet. Just minimization.
Internal for now. Will be public once finished.

src/private/widgets/SideBarWidget_p.h

@@ -0,0 +1,48 @@
+/*
+  This file is part of KDDockWidgets.
+
+  SPDX-FileCopyrightText: 2019-2020 Klarälvdalens Datakonsult AB, a KDAB Group company <info@kdab.com>
+  Author: Sérgio Martins <sergio.martins@kdab.com>
+
+  SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only
+
+  Contact KDAB at <info@kdab.com> for commercial licensing options.
+*/
+
+#ifndef KD_SIDEBARWIDGET_P_H
+#define KD_SIDEBARWIDGET_P_H
+
+
+#include "../../docks_export.h"
+#include "../SideBar_p.h"
+
+QT_BEGIN_NAMESPACE
+class QBoxLayout;
+class QAbstractButton;
+QT_END_NAMESPACE
+
+namespace KDDockWidgets {
+
+class DockWidget;
+class Frame;
+
+class DOCKS_EXPORT SideBarWidget : public SideBar
+{
+    Q_OBJECT
+public:
+    explicit SideBarWidget(Qt::Orientation orientation, QWidget *parent);
+
+protected:
+    void addDockWidget_Impl(DockWidgetBase *dock) override;
+    void removeDockWidget_Impl(DockWidgetBase *dock) override;
+
+    // virtual so users can provide their own buttons
+    virtual QAbstractButton* createButton();
+
+private:
+    QBoxLayout *const m_layout;
+};
+
+}
+
+#endif