Plugins: Add skeleton for new ExtensionManager plugin

This initial commit is merely a mockup of a ExtensionManager concept.

Change-Id: I19b2285667678a86097c043cc27a554545559eff
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/extensionmanager/extensionmanagerwidget.h

@@ -0,0 +1,33 @@
+// Copyright (C) 2023 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
+
+#include "extensionmanagertr.h"
+
+#include "extensionmanagerconstants.h"
+
+#include <coreplugin/welcomepagehelper.h>
+
+QT_BEGIN_NAMESPACE
+class QTextBrowser;
+QT_END_NAMESPACE
+
+namespace ExtensionManager::Internal {
+
+class CollapsingWidget;
+class ExtensionsBrowser;
+
+class ExtensionManagerWidget final : public Core::ResizeSignallingWidget
+{
+public:
+    explicit ExtensionManagerWidget();
+
+private:
+    void updateView(const QModelIndex &current, [[maybe_unused]] const QModelIndex &previous);
+
+    ExtensionsBrowser *m_leftColumn;
+    CollapsingWidget *m_secondarDescriptionWidget;
+    QTextBrowser *m_primaryDescription;
+    QTextBrowser *m_secondaryDescription;
+};
+
+} // ExtensionManager::Internal