provide manual timeout to egui in reactive mode to force refresh even if there's no new input

[coderedart]

Is your feature request related to a problem? Please describe.
egui's reactive mode is really great for applications, but the UI not updating AT ALL is a pain point for many newbies. at present, they are usually told to call request_repaint from another thread. not really user friendly imo.

Describe the solution you'd like
just let users set a optional timeout, and when there's no input for the duration of the timeout, egui does the request repaint thing itself.

Describe alternatives you've considered
I don't see any alternatives.

Additional context
all big three windowing libraries: winit, GLFW and SDL2, have support for wait_event timeout function, where the event loop will yield control back to us when it fails to get any events for the specified timeout. so, it should be pretty clean to implement.

[emilk]

eframe:s web backend already repaints every second by default: https://github.com/emilk/egui/blob/master/eframe/src/web/backend.rs#L413

I agree this should be controllable and also implemented on native.

The simplest would probably be to have a settable frequency of "update at least this often".

The more powerful (but also more complicated) thing to add is to have egui::Context::repaint_in(Duration) to schedule a repaint after a certain amount of time.

Is it something you'd like to work on?