The purpose of this is to expose `frame.storage()` and `frame.storage_mut()` so users can save/load app state from the `App::update` function, without having to add another parameter to that function.
Changes:
* Added `Frame::storage()` and `Frame::storage_mut()`
* `App::update` now takes a `&mut Frame` rather than just `&Frame`
* `Frame` is no longer `Clone` or `Sync` (doesn't have to be since https://github.com/emilk/egui/pull/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`).
