Improve container selection panel UI

[abey79]

What

• Fixes Container selection panel UI has bad spacing #6704
• Can be improved with Introduce ButtonContent for list item #6707

This PR fixes the spacing regression and generally cleans up the Container selection panel UI using list item and the section_collapsing_header. The buttons are implemented using list item as well, which is far from ideal -> #6707

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[Wumpf]

nice! nothing that can't be solved with listitem 😄