[AltStoreCore] Refactors core AltStore logic into AltStoreCore framework

AltStoreCore will contain all shared AltStore code between AltStore and any app extensions. Initially, it includes all AltStore model logic.

AltStoreCore/Extensions/UIApplication+AppExtension.swift

@@ -0,0 +1,17 @@
+//
+//  UIApplication+AppExtension.swift
+//  DeltaCore
+//
+//  Created by Riley Testut on 6/14/18.
+//  Copyright © 2018 Riley Testut. All rights reserved.
+//
+
+import UIKit
+
+public extension UIApplication
+{
+    // Cannot normally use UIApplication.shared from extensions, so we get around this by calling value(forKey:).
+    class var alt_shared: UIApplication? {
+        return UIApplication.value(forKey: "sharedApplication") as? UIApplication
+    }
+}