refactor filetree

[extrawurst]

since https://github.com/extrawurst/gitui/pull/718 landed we have a new more performant/maintainable filetree. it is currently only used for the file-tree-view. we should switch it in for the old statustree aswell (for this we need to extend it to support containing actual data to piggy bag)

• [ ] throw away old tree
• [ ] add scrollbar support to places we use this one

[stale[bot]]

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