[ui] add readOnly property on AttributeEditor

* setting readOnly to 'true' disable editing on AttributeItemDelegate components
* use this property instead of disabling the whole AttributeEditor when graph is being computed

meshroom/ui/qml/main.qml

@@ -293,8 +293,8 @@ ApplicationWindow {
                     sourceComponent: Component {
                         AttributeEditor {
                             node: graphEditor.selectedNode
-                            // Disable editor when computing
-                            enabled: !_reconstruction.computing
+                            // Make AttributeEditor readOnly when computing
+                            readOnly: _reconstruction.computing
                         }
                     }
                 }