Combine the 3 tabs: bands, El. Structure and PDOS into a single top-level band, called “Electronic structure”

[superstar54]

• In the plugin input checkboxes, call the properties “Electronic structure (bands)” and “Electronic structure (PDOS)” to make it clear it’s going to be in the “Electronic structure” output tab in the end.
• If it’s easy, only show one advanced input tab for both called “Electronic structure”: there are not too many advanced options, so you can just put both the PDOS and bands in the same tab. Otherwise OK to keep as it is now, but give the very same name to the tabs as the checkboxes: “Electronic structure (bands)” and “Electronic structure (PDOS)”.
  In the output, only show a single “Electronic structure” results tab.
• As a first simple step, just move three tabs inside this tab, calling them: “Bands only”, “PDOS only”, “Combined bands+PDOS”
  In the future, open an issue (to be then rediscussed for the next release) whether we have no internal tabs, we have only the combined plot, and we simply have some checkbox or similar (e.g. a dropdown?) to decide what to show.