Add support to MultiPack

[mylibrar]

Is your feature request related to a problem? Please describe.
Stave can only support visualization of DataPack. We should extend its support to MultiPack.

Describe the solution you'd like
A MultiPack is a collection of multiple DataPacks, and we need to provide some interfaces for user to specify a DataPack for visualization. A drop down menu/list can be added to select DataPack based on its reference name in MultiPack.

Describe alternatives you've considered
Other design for DataPack selection can be accepted.

Additional context
It's better to include some test cases for MultiPack rendering.