Export just the spectrum as png/svg/pdf

[douweschulte]

It is very hard to just get a specific part of the page. The print (export) now shows the least amount of the page to work. It might be possible to get a specific element with WebRTC video capturing standards.

[douweschulte]

It might be possible to hide everything else that is present on the page during exporting. And it might be worth it to provide two export options, one full (like now but reviewed a bit) and one with just the spectrum (and pepetidoform(s)).
Discussed in #47