Add support to `MultiPack`

**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 `DataPack`s, 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.

No files, tests, or entry points are named. Start by locating Stave's existing DataPack visualization and rendering tests, then determine how MultiPack reference names can drive a DataPack selector; done means a user can choose a DataPack and MultiPack rendering is covered by tests.

Written by the indexing model from the issue text.