Refactor Plot::show_dyn internals

[michalsustr]

Closes #166

show_dyn is a massive function with non-trivial dependencies.

This PR attempts to untangle it and make it easier to navigate and understand.

Public API changes:

• simplified show lifetime

Removed items from the public API

-pub fn egui_plot::Plot<'a>::show<'b, R>(self, ui: &mut egui::ui::Ui, build_fn: impl core::ops::function::FnOnce(&mut egui_plot::PlotUi<'b>) -> R + 'a) -> egui_plot::PlotResponse<R>

Added items to the public API

+pub fn egui_plot::Plot<'a>::show<R>(self, ui: &mut egui::ui::Ui, build_fn: impl core::ops::function::FnOnce(&mut egui_plot::PlotUi<'a>) -> R + 'a) -> egui_plot::PlotResponse<R>

[emilk]

Code LGTM! Thanks for tackling this beast

[emilk]

Future improvement (can save for other PR):

Suggested change

	show_x: show_xy.x,
	show_y: show_xy.y,
	show_xy,