* Re-implement PaintCallbacks With Support for WGPU
This makes breaking changes to the PaintCallback system, but makes it
flexible enough to support both the WGPU and glow backends with custom
rendering.
Also adds a WGPU equivalent to the glow demo for custom painting.
* Remove integration name (it is always eframe)
* Remove egui_web crate
* Move egui_web/CHANGELOG.md into eframe/CHANGELOG.md
* Remove all mentions of egui_web
* Remove epi crate and absorb into eframe
* egui_glow: only use puffin on native
* Remove WASM doc from CI (we don't generate it anyways!)
* Remove eframe::epi and improve eframe docs