update to image 0.20

[rukai]

• Tested on all platforms changed
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created an example program if it would help users understand this functionality

Tested on x11/kde and windows.
This should be merged before the release of 0.18.

[rukai]

Just noticed there was a warning triggered when building the example without the icon_loading feature, so I fixed that as well by moving the use winit::Icon into fn main() { }

[francesca64]

Alright, awesome!