Gui cosmetics

[armengau]

• Color option for the main curves (coadded spectrum and noise, model)
• Improved widget layout when --no_vi_widgets is requested (which is the case of the interface to legacyviewer)
• Added a TextInput to select a spectrum

[coveralls]

coverage: 2.014% (-0.01%) from 2.024%
when pulling c69ef0e on gui-cosmetics
into 445d05d on main.

[armengau]

About the color option: eg. plotspectra(colors=['black', 'blue', 'orange'])
or prospect_pages --colors black blue orange
changes the colors of the coadded spectra, "model", and coadded noise vectors respectively.

• This makes it as simple as possible for the user to choose colors.
• No option to change the colors of single-arm spectra, as their color codes is not arbitrary but associated to blue, red and NIR (plus they're less used)

[weaverba137]

I don't see any issues with this PR. I just had a minor comment on the change log.