* Move window building to egui-winit
* Move icon loading to egui-winit
* `use glow::HasContext;` -> `use glow::HasContext as _;`
* Move FileStorage into epi behind a feature flag
* De-duplicate screen_size_in_pixels and native_pixels_per_point
* Move creation of FileStorage to epi
* Handle epi app output (window size changes etc) in egui-winit
* Move app and memory persistence and autosave logic to egui-winit
* fix check.sh
* Make the epi backend opt-in for egui_glium and egui_glow
* Fix persistence
* Add integration name to epi::IntegrationInfo and the demo
* Clean up Cargo.toml files and fix making egui_glium optional
* fix typo
* Make egui_glium compile without the `epi` feature
Additionally to emmiting egui::Event::Touch events, on_touch() now also
emits egui::Event::PointerButton, egui::event::PointerMoved and
egui::event::PointerGone facilitating UI usage on touchscreen devices.
If there are multiple touches, only the first one will be translated
into pointer events. This prevents situations with two pointers.
