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_demo_lib/Cargo.toml

@@ -39,6 +39,7 @@ epi = { version = "0.17.0", path = "../epi" }
 
 chrono = { version = "0.4", optional = true, features = ["js-sys", "wasmbind"] }
 enum-map = { version = "2", features = ["serde"] }
+tracing = "0.1"
 unicode_names2 = { version = "0.5.0", default-features = false }
 
 # feature "http":