Fix crash when closing export dialog while export is running

TaskDialog::closeEvent() was calling deleteLater() immediately when the
user closed the window. The ExportTask was a child QObject of the dialog,
so it got destroyed while its Run() method was still executing in the
worker thread, leading to a use-after-free on encoder_->Close().

Defer deletion until the QFutureWatcher reports that the task has actually
finished. A new task_finished_ flag tracks this so closeEvent only deletes
when it is safe, and TaskFinished() closes the dialog (which now deletes
instead of calling close() again while the task is still alive).

app/dialog/task/task.h

@@ -75,6 +75,8 @@ private:
 
 	bool already_shown_;
 
+	bool task_finished_;
+
 private slots:
 	void TaskFinished();
 };