Added interaction control fields to PlotUi

[jbuckmccready]

This is a WIP for closing emilk/egui_plot#2. I changed the code to demonstrate what I describe in the issue regarding allow_drag and drag editing points. I'm creating this draft before continuing my work to get feedback regarding:

1. Should I move the fields to PlotUi (breaking API change) or duplicate them to be on both Plot and PlotUi (non-breaking API change)? Or perhaps take a different approach entirely?
2. I added a demo just to show editing (by dragging) points. This demo could be merged with the plot interactive demo already in place (since that interactive demo plot is currently left empty), left as its own demo, or removed entirely if deemed not useful.

[emilk]

egui_plot has recently been moved to its own repository, at https://github.com/emilk/egui_plot

This will hopefully speed up its development by having more reviewers and maintainers.

Please re-open this PR at https://github.com/emilk/egui_plot/pulls

See also:

• Looking for maintainer egui_plot#58
• Move egui_plot to its own repo #4828