Plot point userdata rebase

[bircni]

Continuement of #76

[github-actions]

View snapshot changes at kitdiff

[bircni]

Hi @kitizz, thanks for the detailed review—here are the follow-ups:

• examples/userdata_points/src/app.rs: I scrapped the Arc<Mutex<HashMap<Id, …>>> dance and now pre-populate an
  egui::IdMap keyed by the item ids before calling Plot::show. The formatter no longer relies on show() executing first,
  the item argument is now Some((Id, index)) when hovering actual plot points, and None otherwise (so the item argument
  can reliably be used just like you suggested).
• egui_plot/src/label.rs + egui_plot/src/plot.rs: Added docs describing that the third formatter argument carries the
  hovered item‘s (Id, index) when available and is None for empty space. The example also names that argument so it’s
  clear how to format the final tooltip text.