refactor: move file open event handling to standalone file

app/fileopeneventhandler.h

@@ -0,0 +1,21 @@
+// SPDX-FileCopyrightText: 2024 Gary Wang <git@blumia.net>
+//
+// SPDX-License-Identifier: MIT
+
+#pragma once
+
+#include <QObject>
+
+class FileOpenEventHandler : public QObject
+{
+    Q_OBJECT
+
+public:
+    explicit FileOpenEventHandler(QObject *parent = nullptr);
+
+protected:
+    bool eventFilter(QObject *obj, QEvent *event) override;
+
+signals:
+    void fileOpen(const QUrl &url);
+};