Get ready for 0.4

[Lokathor]

We've got some breaking changes building up and it's time to move to a 0.4

• TODO: There's some docs in the related PR from Thomcc that we should incorporate here.
• TODO: we need to rename ios to uikit and macos to appkit, since you can use the macos types on target_os="ios" (via catalyst) and the ios types on target_os!="ios" (tvos, for example)

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• Closes Proposal: Remove all the cfg guards. #63
• Closes Use separate types for each platform #60
• Closes New release?  #58

[DJMcNab]

Maybe a better name for TrustedRawWindowHandle would be something like StableRawWindowHandle or similar, since the guarantee is that the inner handle won't be mutated.

[i509VCB]

I'm curious here, I've noticed some of the handles like macOS, Windows, Android and iOS use Option<NonNull<c_void>> while Wayland, XCB and Xlib still use *mut c_void. Is this inconsistency intentional or were those specific handles not changed yet to be in line with the other handles.

[Lokathor]

I think it was an accidental oversight.

However, I also think that it's a bad change in retrospect. Every pointer here can optionally be null, that's just a blanket rule. I think actually using Option<NonNull<c_void>> makes things less ergonomic though. Realistically, we'll probably want to undo that change before the next release.

[Lokathor]

Merging this partial progress, and the rest will be finished up in #70