qt, refactor: Move InitExecutor class into its own module

This change makes InitExecutor class re-usable by an alternative GUI,
e.g., QML-based one.

src/qt/initexecutor.h

@@ -0,0 +1,46 @@
+// Copyright (c) 2014-2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_QT_INITEXECUTOR_H
+#define BITCOIN_QT_INITEXECUTOR_H
+
+#include <interfaces/node.h>
+
+#include <exception>
+
+#include <QObject>
+#include <QThread>
+
+QT_BEGIN_NAMESPACE
+class QString;
+QT_END_NAMESPACE
+
+/** Class encapsulating Bitcoin Core startup and shutdown.
+ * Allows running startup and shutdown in a different thread from the UI thread.
+ */
+class InitExecutor: public QObject
+{
+    Q_OBJECT
+public:
+    explicit InitExecutor(interfaces::Node& node);
+    ~InitExecutor();
+
+public Q_SLOTS:
+    void initialize();
+    void shutdown();
+
+Q_SIGNALS:
+    void initializeResult(bool success, interfaces::BlockAndHeaderTipInfo tip_info);
+    void shutdownResult();
+    void runawayException(const QString &message);
+
+private:
+    /// Pass fatal exception message to UI thread
+    void handleRunawayException(const std::exception *e);
+
+    interfaces::Node& m_node;
+    QThread m_thread;
+};
+
+#endif // BITCOIN_QT_INITEXECUTOR_H