Add markdown viewer plugin

Fixes: QTCREATORBUG-27883
Change-Id: Ie4099b8a322797d300bc15a80333bcb361ecafc3
Reviewed-by: André Hartmann <aha_1980@gmx.de>

src/plugins/markdownviewer/markdownviewerwidget.cpp

@@ -0,0 +1,58 @@
+// Copyright (C) 2023 Tasuku Suzuki
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
+
+#include "markdownviewerwidget.h"
+#include "markdowneditor.h"
+#include "markdownbrowser.h"
+
+#include <texteditor/textdocument.h>
+
+namespace Markdown {
+namespace Internal {
+
+struct MarkdownViewerWidgetPrivate
+{
+    MarkdownBrowser *browser;
+    MarkdownEditor *editor;
+};
+
+MarkdownViewerWidget::MarkdownViewerWidget()
+    : d(new MarkdownViewerWidgetPrivate)
+{
+    d->browser = new MarkdownBrowser(this);
+    d->browser->setOpenExternalLinks(true);
+    d->browser->setFrameShape(QFrame::NoFrame);
+    d->editor = new MarkdownEditor(this);
+
+    connect(d->editor->document(), &QTextDocument::contentsChanged,
+            this, [this]() {
+        const auto plainText = d->editor->textDocument()->plainText();
+        d->browser->setMarkdown(plainText);
+    });
+}
+
+MarkdownViewerWidget::~MarkdownViewerWidget()
+{
+    delete d;
+}
+
+Core::IDocument *MarkdownViewerWidget::document() const
+{
+    return d->editor->textDocument();
+}
+
+bool MarkdownViewerWidget::isEditorVisible() const
+{
+    return d->editor->isVisible();
+}
+
+void MarkdownViewerWidget::setEditorVisible(bool editorVisible)
+{
+    if (d->editor->isVisible() == editorVisible)
+        return;
+    d->editor->setVisible(editorVisible);
+    emit editorVisibleChanged(editorVisible);
+}
+
+} // namespace Internal
+} // namespace Markdown