Add collapsable sidebar item

[mabijkerk]

This is an enhancement idea for etmodel. When I click on a sidebar item that has many tabs, for example "Cost & Efficiencies", some sidebar items may fall from my screen. In the example below, "Results & data" is no longer visible:

I think it would be useful if you could click on the sidebar item that you have currently open, and by doing so, would make it collaps. What do you think @Charlottevm @noracato?

[github-actions]

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