De-clutter the 'Model' tab

[aalfons]

With the (at the moment still hacked-together) functionality to draw the model diagram for the selected model and variables, the Model tab is rather cluttered with the additional explanations of the different models. There are some options:

1. There could be something like a help button. If we click the button, the explanations of the different mediation models pop up. Are there nice looking, pre-defined help buttons in shiny?
2. We could split up the main panel into different tabs: one tab for the model diagram, and another tab for the explanations.

What should we do? I have a preference for a help button, because I think in the long run it would be good to have a help button with explanations on each tab to guide users (see also #24).

[aalfons]

Have a look at showTab() and hideTab() together with an observer and a logical reactive value.

[AuroreAA]

https://shiny.posit.co/r/reference/shiny/1.4.0/showtab