Add button 👁 to show code of cells

[dorn-gerhard]

Currently in PlutoSliderServer notebooks, the code of cells is not switchable - you only see it, if it the code is shown in the basic notebook.

It would be cool to add the possibility to show the code of the cells.
There are some implementation possibilities - I am curious what you think about them:

• add an option to make all cells switchable
• add an option per cell to make it switchable (either as button or hidden comment property), so some cells can reveal code, others not

@ctrekker maybe you already have something like this in mind?