Better design of `_StructureDataBaseViewer` and `StructureDataViewer`

As a developer, one would like to have a clear separation of concern between `_StructureDataBaseViewer` and `StructureDataViewer` allowing for easy further improvements and modifications.

The base class is meant to provide a common functionality of structure viewer and (yet to come) trajectory viewer. Unfortunately, due to some design flows it cannot be easily done with the current implementation.

Herein, after a meeting with @unkcpz and @danielhollas, we agreed to have a closer look at those two classes and come up with a better design.

## Known issues

1. Both `structure` and `displayed_structure` traits are defined in the `StructureDataViewer` class but used in `_StructureDataBaseViewer`.
2. Point 1. also makes it impossible to use `_StructureDataBaseViewer` as the base class for the trajectory viewer.
3. An addition of new displayed properties requires new traits to `_StructureDataBaseViewer`, which is not optimal as most of the features are stored in the `structure` trait.

Pinging @danielhollas and @unkcpz for additional input.