Remove image dependency from eframe (currently necessary for IconData)

[valadaptive]

Currently, eframe has a mandatory dependency on the image crate. This is because of the eframe::icon_data::from_png_bytes API.

This could probably be replaced with the ImageLoader API 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<IconData> for ColorImage treats it like it is premultiplied, so that code in general should probably be cleaned up.