Add slots to Tree component

[kinow]

These changes partially address #145

WIP: hard/boring part done and working, which was converting each element to a JSX JS call. Now pending:

• merge conflicts after Add families and parents to tree view query #291 (new block added for families needs to be converted to JSX too)
• insert slots and pass them recursively to the TreeItem child component
• implement the GScan sidebar panel using the Tree with minDepth=0 & TreeItems with slots for the top level workflow level, which do not render children nodes, instead displaying only workflow name, and summary of existing job states in the workflow

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Already covered by existing tests.
• No change log entry required (why? e.g. invisible to users).
• No documentation update required.

[kinow]

Should have the exact same behaviour as previous version when seeing the tree view/component.

[kinow]

Not necessary, superseded by simpler GScan component in #301