The value traitlets of WorkChainSelector

This is one of the widgets in QeApp that we want to move to the AWB, since it is reused by copy paste in also SSSPapp and @danielhollas's ispg app. I just find in the QeApp, the doc is not correct, and the `value` better to be a str so can accept UUID which is now used for other process widget as the traitlets type.

https://github.com/aiidalab/aiidalab-qe/blob/2e4c7faa50240071e80d855969a49e0643154412/aiidalab_qe/process.py#L13-L14