Add support for Mac Catalyst, PSPDFKit 10.1 for iOS, iOS 14, and Xcode 12.2

[radazzouz]

Details

This PR updates the PDFXKit project to add support for Mac Catalyst, PSPDFKit 10 for iOS, iOS 14, and Xcode 12.

However, since Carthage does not support XCFrameworks at all, we're removing the Carthage integration.

Only the manual setup and the CocoaPods integrations will be supported.

This PR also updates PDFXKitExample, making it a separate Xcode project which uses the resulting XCFramework, rather than the fat framework from PDFXKit project. We do this to support Mac Catalyst.

• The README is updated.
• The PDFXKitExample app works great on iOS (both simulator and device) and Mac Catalyst.
• rake compile works
• The CocoaPods integration works in a sample project

[matej]

Tested local builds, xcframework generation and CocoaPods. It all seems to work nicely. The only thing that stood out are the CocoaPods instructions.

[matej]

Is the dependency now queried implicitly? The docs above still say "You'll have to add PSPDFKit as well as PDFXKit as a dependency to your Podfile.". We either should adjust that or keep pod 'PSPDFKit' (but we no longer need the URL).

[radazzouz]

Good point! The integration is now implicit because PSPDFKit is a dependency in the podspec (https://github.com/PSPDFKit/PDFXKit/pull/9/files#diff-6f2cb93d716e8580740436aa50d12b4ca997d27f1ebe7c02ea1ba8e5d4d1a47b).

I'll add back the pod (with the URL to be explicit) in the README.

[matej]

We could also just change the description (i.e., remove "You'll have to add PSPDFKit as well as PDFXKit as a dependency to your Podfile") and keep the simpler form.