CLI: disable async crash-recovery backup in headless mode (fixes segfault)

QETProject schedules an asynchronous crash-recovery backup on construction
(writeBackup() -> QtConcurrent::run(QET::writeToFile, ..., &m_backup_file)).
In one-shot CLI mode the QETProject is destroyed as soon as the command
returns, while that background write still references its m_backup_file
member — an intermittent use-after-free segfault during teardown (~1 in 6
runs; observed on --resave and --set-titleblock).

A crash-recovery backup is meaningless for a short-lived headless command,
so add QETProject::setBackupEnabled(false), called from the CLI entry in
main(). writeBackup() then early-returns, so no background write is ever
launched. Fixes the crash for all CLI commands. See #492.

sources/qetproject.cpp

@@ -43,6 +43,13 @@
 
 static int BACKUP_INTERVAL = 1200000; //interval in ms of backup = 20min
 
+bool QETProject::m_backup_enabled = true;
+
+void QETProject::setBackupEnabled(bool enabled)
+{
+	m_backup_enabled = enabled;
+}
+
 /**
 	@brief QETProject::QETProject
 	Create a empty project
@@ -1783,6 +1790,8 @@ void QETProject::addDiagram(Diagram *diagram, int pos)
 */
 void QETProject::writeBackup()
 {
+	if (!m_backup_enabled)
+		return;
 #ifdef BUILD_WITHOUT_KF5
 #else
 #	if QT_VERSION < QT_VERSION_CHECK(6, 0, 0) // ### Qt 6: remove