Context::request_repaint will wake up the UI thread (#1366)

This adds a callback (set by `Context::set_request_repaint_callback`)
which integration can use to wake up the UI thread.

eframe (egui_web and egui_glow) will use this, replacing
`epi::Frame::request_repaint`.

Existing code calling `epi::Frame::request_repaint` should be changed
to instead call `egui::Context::request_repaint`.

This is the first callback added to the egui API, which otherwise is
completely driven by data.

The purpose of this is to remove the confusion between the two
`request_repaint` methods (by removing one). Furthermore, it makes
`epi::Frame` a lot simpler, allowing future simplifications to it
(perhaps no longer having it be `Send+Sync+Clone`).

egui-winit/src/epi.rs

@@ -233,7 +233,6 @@ impl EpiIntegration {
         max_texture_side: usize,
         window: &winit::window::Window,
         gl: &std::rc::Rc<glow::Context>,
-        repaint_signal: std::sync::Arc<dyn epi::backend::RepaintSignal>,
         persistence: crate::epi::Persistence,
         app: Box<dyn epi::App>,
     ) -> Self {
@@ -252,7 +251,6 @@ impl EpiIntegration {
                 native_pixels_per_point: Some(crate::native_pixels_per_point(window)),
             },
             output: Default::default(),
-            repaint_signal,
         });
 
         if prefer_dark_mode == Some(true) {