Sync egui_plot item visibility with entity visibility

[Wumpf]

Annoyingly there's two types of visibility right now in the plot: egui_plot "hidden item" (PlotMemory::hidden_items) and our regular visibility flag. The former is set by clicking on the legend in egui_plot, the later via our regular api.

To clean this up the hidden items need to be accessible in egui plot. Note that my last change on egui plots here

• Plot items now have optional id which is returned in the plot's response when hovered emilk/egui#3920

actually made this a bit of a mess now: This change introduces ids that can be optionally set, but internally plot items are uniquely identified by name! We should use the id everywhere but automatically derive it from the name if not specified otherwise.