Add a big grey text when nothing is loaded

[lpatiny]

If there is nothing loaded add a text

'Drag & drop here a Jcamp or NMRium file'

We will add more file formats later.

There could be some styling showing it is a drop zone like in this example

[hamed-musallam]

@lpatiny where you need to display it, at molecular panel?

[lpatiny]

In the middle and in big instead of the spectrum (that is not present anyway)

[lpatiny]

Actually the scale could be hidden as well. We don't know anyway if it will be ppm or time.