Clean up macOS class declaration

[madsmtm]

Part of #2427.

Use the new objc2::declare_class! macro to vastly improve our code that declares custom classes. Notable improvements:

• The selector and the method implementation are right next to each other (instead of being split out over the file).
• You can use self very naturally
• You don't have to worry about ABI details like the extra Sel parameter
• You can now take and return bool directly in delegate methods

Further improvements (e.g. to ivar handling) will come later. Will also fix the formatting of view.rs and window_delegate.rs in a subsequent PR, to keep the diff smaller.

• Tested on all platforms changed