Fix #492: wait for async backup before destroying QETProject

writeBackup() fires QtConcurrent::run(QET::writeToFile, ..., &m_backup_file)
fire-and-forget: the QFuture was discarded and nothing kept m_backup_file
alive until the worker finished. If the QETProject was destroyed first, the
worker wrote through the freed member -> use-after-free crash in
QET::writeToFile (intermittent; ~1/6 on short-lived CLI runs).

Store the QFuture and waitForFinished() in ~QETProject (and before
setFilePath() re-points the managed backup file). Also skip launching a new
backup while one is still running, so two threads never write m_backup_file
at once.

The Qt6 path is still a TODO stub and the QtConcurrent block is KF5-only, so
this affects only the Qt5/KF5 build that actually has the backup code.

sources/qetproject.h

@@ -35,6 +35,7 @@
 #endif
 
 #include <QHash>
+#include <QFuture>
 
 class Diagram;
 class ElementsLocation;
@@ -295,6 +296,7 @@ class QETProject : public QObject
 		bool m_freeze_new_conductors = false;
 		QTimer m_save_backup_timer,
 			   m_autosave_timer;
+		QFuture<bool> m_backup_future;
 #ifdef BUILD_WITHOUT_KF5
 #else
 		KAutoSaveFile m_backup_file;