Remove icon loading feature

[Osspial]

• 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

Removes the icon_loading feature from the eventloop-2.0 branch. This does not remove window icons as a feature - it just removes the image crate integration. This is because we've gotten so many issues/prs about the version of the image crate that maintaining our support for it seems to be far more trouble than it's worth. Not only that, but keeping the dependency will just lock us into a never-ending series of breaking changes every time image wants to bump its major version number - preventing us from ever stabilizing until image reaches 1.0, and potentially forcing a major bump after that if image ever decides to make more breaking changes after that.

Yes, this PR forces users to add some boilerplate code, but it really isn't that much.

Discussion is encouraged, of course, but I've got a strong preference towards merging this. Closes #661.

[Osspial]

There seems to be overwhelming support for this so I'll go ahead and merge it.