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`).
