[structure view] fine-tune icons for tree nodes

[martinlippert]

Issue to discuss and collect changes to the icons of the structure tree nodes.

Icons should be selected from the second table on https://code.visualstudio.com/api/references/icons-in-labels#icon-listing

(cc @odrotbohm)

[martinlippert]

Fixed via ba1701a, 298b8f4, 2735128