[ui] GraphEditor: consistent readOnly mode when computing

* lock edit actions when graph is being computed
* add an advanced option to control this behavior and unlock it (stored in persistent settings)

meshroom/ui/qml/GraphEditor/GraphEditorSettings.qml

@@ -8,4 +8,5 @@ import Qt.labs.settings 1.0
 Settings {
     category: 'GraphEditor'
     property bool showAdvancedAttributes: false
+    property bool lockOnCompute: true
 }