Trailing section inset on orthogonal views

[notjosh]

~(Cherry-picked #80 so the example builds)~ rebased! 💅

Part of #78.

Adds the section insets to the end of an orthogonal view in the direction it scrolls.

I didn't spend time checking why the vertical insets aren't applied - I don't think it's the same thing as here, but it's related.

Before:

After:

iOS 13 reference:

[notjosh]

I didn't spend time checking why the vertical insets aren't applied - I don't think it's the same thing as here, but it's related.

Ah, this seems like it could be fixed here: IBPUICollectionViewCompositionalLayout.m#L236, updating it to:

- frame.size.width = CGRectGetWidth(cellFrame);
+ frame.size.width = CGRectGetWidth(cellFrame) + layoutItem.contentInsets.trailing;

[kishikawakatsumi]

I think section insets should be added in processing sections, not items. This PR is superseded by https://github.com/kishikawakatsumi/IBPCollectionViewCompositionalLayout/pull/85

[kishikawakatsumi]

Thank you for reporting. It is very helpful to fix the issue 😄