Label Loading Placeholder

[WenheLI]

After landing #3, the framework can load and generate labels asynchronously.
Therefore, we need to take care of a placeholder right now.
The proposed solution is:

1. Add a new state/event indicate the loading/generating stage of the label.
2. In this stage, the framework should render or display a placeholder.
3. We might also want to allow a callback/hook to be passed, in case, users need to do customization.