Adds initial AltBackup app

When deactivating an app, AltStore will first install AltBackup in its place. This allows AltBackup to access the (soon to be) inactive app’s sandbox, and backup all files to a shared app group with AltStore. Later when activating, AltStore will again install AltBackup and use it to restore files before installing the actual app again.

AltKit/Bundle+AltStore.swift

@@ -38,4 +38,8 @@ public extension Bundle
         let infoPlistURL = self.bundleURL.appendingPathComponent("ALTCertificate.p12")
         return infoPlistURL
     }
+    
+    var appGroups: [String] {
+        return self.infoDictionary?[Bundle.Info.appGroups] as? [String] ?? []
+    }
 }