Deal with no file loaded

If we drag and drop a gif or a pdf nothing happens. This is quite OK but may be frustrating for a end-user that does not understand what is going on.

I would suggest that if nothing if found we give a dialog.

```
No NMR data detected
We couldn’t find any NMR spectrum in the file you provided.
Please make sure the file contains valid NMR data (see [supported formats](https://docs.nmrium.org/20_1d-spectra/loading_a_spectrum/)
) and try again.
```

Also if a molecule was loaded it would be nice to open the molecule panel if available (and the spectra panel if there are spectra).