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`).

Cargo.lock

@@ -1053,6 +1053,7 @@ dependencies = [
  "poll-promise",
  "serde",
  "syntect",
+ "tracing",
  "unicode_names2",
 ]
 
@@ -1091,6 +1092,7 @@ dependencies = [
  "glow",
  "glutin",
  "memoffset",
+ "parking_lot 0.12.0",
  "tracing",
  "wasm-bindgen",
  "web-sys",