`BandsPdosWidget`: Group sites by chemical element

Discussed with @AndresOrtegaGuerrero

In the current version of the `BandsPdosWidget`, it's only possible to group by kinds, e.g., `Ni1` and `Ni2` in case of an antiferromagnetic simulation of Ni. However, it would be great to have the possibility to group by the actual chemical element, in the previous case simply `Ni`. QE requires you to define the different groups, however, in reality, you might not be interested in the different groups.

This additional category should only be added to the dropdown in case tags are detected. Hence, not only the additional category but also some additional logic should be added.