Add function to request a repaint to WebRunner

[AlexanderSchuetz97]

Is your feature request related to a problem? Please describe.
I want to integrate egui into an existing large singlepage application written in javascript/typescript.
The javascript/typescript app may change the state of the application outside of what rust/egui is aware.
(I.e. for example I use egui to visualize some data and the js part changes the data)
Rust would have to be notified somehow that a repaint is required.

Describe the solution you'd like
It would be easiest to just be able to call WebRunner::request_repaint or something similar.
This would/should cause the Apps update routine to be called without any mouse/input events on the canvas
causing rust to become aware of any changes in state and updating the UI accordingly.
To call this from JS, I could then expose a fn to javascript that would call WebRunner::request_repaint.

Describe alternatives you've considered
A workaround exists as the egui::Context object has a repaint method and the WebRunner has the app.
First I added the egui::Context as a member to my App Struct (Thankfully its just an Arc to a Context delegate!)
Then on the WebRunner in the exposed fn I call

let runner : &WebRunner = ....... 
runner.app_mut::<MyAppStructNameHere>().map(|app| app.repaint())

In the impl of my App:

pub fn repaint(&self) {
    self.ctx.request_repaint();
}

As you can see this is not a very intuitive way to do this.

Additional context
There may be a more obvieus, easy/intended way of doing this, that I just haven't seen.
If that is the case then I am sorry for wasting your time.