Remove `image` dependency from `eframe` (currently necessary for `IconData`)

Nobody has claimed this yet.

Currently, eframe has a mandatory dependency on the `image` crate. This is because of [the `eframe::icon_data::from_png_bytes` API](https://docs.rs/eframe/latest/eframe/icon_data/fn.from_png_bytes.html).

This could probably be replaced with [the `ImageLoader` API](https://docs.rs/egui/0.30.0/egui/load/trait.ImageLoader.html) and `ImageSource`, but there are a couple wrinkles in the API:

- It conflates image *loading* with image *decoding*; as far as I can tell, images are just blank until they're done asynchronously loading. This may not be what we want; we should probably set the window icon *before* starting the rest of the application.
- All other egui image APIs (e.g. `ColorImage`) return *premultiplied* images, whereas `IconData` stores non-premultiplied images. I'm not actually sure if that's correct for all platforms, but that's the current state of things. Also, `From for ColorImage` treats it like it *is* premultiplied, so that code in general should probably be cleaned up.

Start by reading eframe::icon_data::from_png_bytes and the egui ImageLoader, ImageSource, ColorImage, and IconData APIs. Resolve whether loading and decoding should remain synchronous for window icons and clarify premultiplied versus non-premultiplied pixels, including From for ColorImage. Done means eframe no longer requires image while icon handling remains correct across platforms.

Written by the indexing model from the issue text.