👌 IMPROVE: Structure selection

Several small comments from @giovannipizzi on the Structure selection step:

- [ ] Was the table with atomic coordinates removed? (to decide whether to put them back)
- [x] The POVRAY renderer does not seem to take into account the supercell settings. (https://github.com/aiidalab/aiidalab-widgets-base/issues/252)
- [ ] When rendering, is it possible to run in the background, so it does not block the interaction with the rest of the notebook? (or warn with an "alert" that this will take some seconds and block?) (#253)
- [ ] Every time I press the screenshot or Render button, I get a message from my browser (Safari) that the page is going to change (see attachment screenshot1). Do you know why?
- [ ] In Safari, the download screenshot downloads a file called "Unknown" without extension, so it's not obvious it's a PNG
- [ ] I would add at least the XSF format for the download (https://github.com/aiidalab/aiidalab-widgets-base/issues/251).
- [ ] one should clarify if the supercell is just for visualisation or also for the simulation (I had this doubt)