Refactoring a QML element into its own file should retain id

With the "Move component into filename.qml" refactoring operation, the user is often chopping up QML files that have grown too large into smaller components. Currently, this refactoring does not keep the id, but instead produces an empty element. This often breaks QML files, because other elements refer to the refactored element by its id.

If the id could be kept in the instance declaration of the component, this would not be a problem.