Fix Swift PM compatibility by creating a separate ObjC target

[asolovev]

Swift Package Manager compatibility got broken after adding Objective-C source code #415 . There are multiple ways to fix it:

• Converted Objc extension to Swift #422 and replace objc UIImage Gif extension with swift uiimage gif version #424 fix it by converting Objective-C extension to Swift one to eliminate mixed source code language.
• This PR separates Objective-C code into a separate folder and target without actual changes to how Gifs are implemented.

While I think making this project Swift-only is better, I made this PR to just fix the Swift Package Manager while keeping the implementation intact.

Other changes are targeted to fix the bundled resources that were not available in the Swift Package #400 . And to make sure that the framework can still be build from the project file (conditional imports are used for Swift PM or project compilation).

For me it has solved the Swift Package Manager problems, but I didn't check if CocoaPods and Carthage work. I did check that the project file allows to build the framework though.

[rserentill]

@suzuki-0000 can we merge this in order to use SKPhotoBrowser with SPM?

[suzuki-0000]

@rserentill thank you for the support 👍 👍
you can merge this. (let me merge at this time