Improved plot interaction methods

[EmbersArc]

#766 has just been merged and is usable, I'd like to add the following features next:

• Access to Ui (or the context?) inside the closure as well. Either through the plot_ui struct or as a second argument.
• A plot_to_screen conversion, to enable custom drawing in screen coordinates based on items in the plot.
• A way to get the current plot bounds.
• Fix the plot_from_screen function. Pretty sure I messed up hard when writing it and it likely does not work as expected right now.

[emilk]

Access to Ui (at least to a &mut Ui) seems like a dangerous footgun to me, as that will invariable lead to users doing stuff like

Plot::new(…).show(|ui, plot_ui| {
    ui.label("Where will this end up?");
});

I think a plot_ui.ctx() accessor to get a &CtxRef makes sense though.