Non-intuitive icons and zoom function

The **icon used for zooming out is not very intuitive** as most software uses the "magnifying glass" with a "+" inside to zoom in and a "-" to zoom out.

Furthermore, while you can zoom in multiple times by repeatedly clicking the "+" icon at the top, **it currently is not possible to go back just one zoom level** to the previous one, which would however be a highly desirable feature.

Furthermore, the "baseline icon" highlighted in blue at the bottom only allows you to switch between baseline at the bottom or in the middle of the spectrum window. It would be very useful if the **spectrum could be freely moved along the vertical (intensity) axis by just dragging it up or down with the mouse**.