Rename GuestInterface to Widget

So we can use it to abstract the host too, not just the guest.

src/private/multisplitter/Widget_qwidget.cpp

@@ -0,0 +1,63 @@
+/*
+  This file is part of KDDockWidgets.
+
+  Copyright (C) 2018-2020 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com
+  Author: Sérgio Martins <sergio.martins@kdab.com>
+
+  This program is free software; you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 2 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include "Widget_qwidget.h"
+#include "Item_p.h"
+
+#include <QDebug>
+
+using namespace Layouting;
+
+QSize Widget_qwidget::minSize() const
+{
+    return widgetMinSize(asWidget());
+}
+
+QSize Widget_qwidget::maxSize() const
+{
+    return asWidget()->maximumSize();
+}
+
+QSize Widget_qwidget::size() const
+{
+    return asWidget()->size();
+}
+
+void Widget_qwidget::setParent(QObject *parent)
+{
+    if (parent && !parent->isWidgetType()) {
+        qWarning() << Q_FUNC_INFO << "parent is not a widget, you have a bug" << parent;
+        Q_ASSERT(false);
+        return;
+    }
+
+    m_thisWidget->setParent(qobject_cast<QWidget*>(parent));
+}
+
+QSize Widget_qwidget::widgetMinSize(const QWidget *w)
+{
+    const int minW = w->minimumWidth() > 0 ? w->minimumWidth()
+                                           : w->minimumSizeHint().width();
+
+    const int minH = w->minimumHeight() > 0 ? w->minimumHeight()
+                                            : w->minimumSizeHint().height();
+
+    return QSize(minW, minH).expandedTo(Item::hardcodedMinimumSize);
+}