Add support for macOS Catalyst (#3698)

* Add check if target environment is macOS Catalyst

* Add changelog entry

CHANGELOG.md

 # master
-*Please put new entries at the top.
+1. Don't include code which uses unavailable classes (like `NSSlider`) when targeting macOS Catalyst (#3698, kudos to @nkristek)
 
 # 10.2.0
 1. Update ReactiveSwift to 6.2.

ReactiveCocoa/AppKit/ActionProxy.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 import ReactiveSwift
 

ReactiveCocoa/AppKit/AppKitReusableComponents.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 import ReactiveSwift
 

ReactiveCocoa/AppKit/NSButton.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 import ReactiveSwift
 

ReactiveCocoa/AppKit/NSCollectionView.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 import ReactiveSwift
 

ReactiveCocoa/AppKit/NSControl.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 import ReactiveSwift
 

ReactiveCocoa/AppKit/NSImageView.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 import ReactiveSwift
 

ReactiveCocoa/AppKit/NSPopUpButton.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 import ReactiveSwift
 

ReactiveCocoa/AppKit/NSSegmentedControl.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 import ReactiveSwift
 

ReactiveCocoa/AppKit/NSSlider.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 import ReactiveSwift
 

ReactiveCocoa/AppKit/NSTableView.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 import ReactiveSwift
 

ReactiveCocoa/AppKit/NSTextField.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 import ReactiveSwift
 

ReactiveCocoa/AppKit/NSTextView.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 import ReactiveSwift
 

ReactiveCocoa/AppKit/NSView.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 import ReactiveSwift
 

ReactiveCocoa/Shared/NSLayoutConstraint.swift

 #if !os(watchOS)
 import ReactiveSwift
 
-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import AppKit
 #else
 import UIKit

ReactiveCocoaTests/AppKit/ActionProxySpec.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import Foundation
 import Quick
 import Nimble

ReactiveCocoaTests/AppKit/AppKitReusableComponentsSpec.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import Quick
 import Nimble
 import ReactiveSwift

ReactiveCocoaTests/AppKit/NSButtonSpec.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import Quick
 import Nimble
 import ReactiveSwift

ReactiveCocoaTests/AppKit/NSCollectionViewSpec.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import Quick
 import Nimble
 import ReactiveCocoa

ReactiveCocoaTests/AppKit/NSControlSpec.swift

-#if canImport(AppKit)
+#if canImport(AppKit) && !targetEnvironment(macCatalyst)
 import Quick
 import Nimble
 import ReactiveSwift