This PR is some "refactor" of the Spectrum code. There is a lot more of work to do, but is better do it by parts.

This PR changes:

• Some refactor/rename of code
• Adds info in the help dialog about pressing the shift + mouse in the Spectrum (draws a line with the actual frequency of the mouse)
• Changes the way the notch appears in the Spectrum. Now is a pyramid that is more intuitive for a notch.
• Changes colors, line width.
• Removes gradient from the notch. It was strange to have a gradient only in the notch. I will look more in depth how it ends adding a gradient to all the filters, but maybe it ends with a mess of colours only.