Implement dataset list viewer

[BECATRUE]

Feature you want to implement

As a new frame of data viewer app, I will implement a dataset list viewer like the explorer app.

How the feature is implemented

How to update datasets

In dataset viewer, all datasets are already fetched on real-time, thus I need to show them only.

How to organize the hierarchy of datasets

Like artiq-dashboard, I will use a dot as a delimiter to separate folders.

How to react when a dataset is clicked

If the dataset is compatible with the original viewer, i.e. it is a 2D or 3D data with a valid format, it will be shown in the original viewer.
Otherwise, it will be shown in a table viewer.