Changes in `eframe`/`egui` between 0.22 and 0.28.1:
- all the plotting stuff in `eframe::egui` got split off into its own
  crate, `egui_plot`
- most native UI options defined as top-level fields in
  `eframe::NativeOptions` got moved into the `viewport` field, which
  uses `egui::viewport::ViewportBuilder`
- the `eframe::Frame::close` method was removed

Changes here:
- add `egui_plot` as a new dependency, looks like all the widgets work
  the same as before
- move all UI options into the appropriate `ViewportBuilder`
- use a workaround for closing the viewport on `ctrl+c` or `q` based on
  sending a `ViewportCommand::Close` to the egui context; this
  apparently has to be done from a separate thread, which isn't great