Increase control over plot widget inside of show method

I would like to be able to control as many aspects of the plot widget as possible from inside the `Plot::show` method to allow deciding whether to allow dragging or zooming based on the pointer position in the plot (and the event state, but that is already accessible from the `PlotUi::ctx`). See my original comment on this here: https://github.com/emilk/egui/issues/759#issuecomment-1013767585

I have added `allow_drag` to the `PlotUi` structure which seems to be working good (although I need to test it more thoroughly for what I am wanting to do). I will look into adding the other viable control functions to it as well (e.g., `allow_zoom`, `show_x`, `show_y`, `show_background`, etc.), I am thinking that anything that is used after the call to the `build_fn` in the `Plot::show` method should be made available for consistency and flexibility. I will make a pull request when I've got everything added, cleaned up, and tested.

@emilk @EmbersArc Do you have any particular design input on this? The simplest non-breaking API change I can see is to just add each of the fields such as `allow_drag`, `allow_zoom`, etc. to the `PlotUi` structure (and add associated methods to mutate them), and default their values to match the fields on the `Plot` structure (so if they are only set on the Plot structure then it is honored), then in the `build_fn` they can be updated. This is somewhat awkward since there are then duplicate methods used to control the plot (one for the `Plot` structure and one for the `PlotUi` structure), would it be better to just move the fields and methods so they are only on the `PlotUi` structure (API breaking change)? Or maybe there's another better way to get at what I'm wanting to add?