Improves error message when registering app + app extension after App ID limit is reached

AltStore/Extensions/ALTApplication+AppExtensions.swift

@@ -0,0 +1,29 @@
+//
+//  ALTApplication+AppExtensions.swift
+//  AltStore
+//
+//  Created by Riley Testut on 2/10/20.
+//  Copyright © 2020 Riley Testut. All rights reserved.
+//
+
+import AltSign
+
+extension ALTApplication
+{
+    var appExtensions: Set<ALTApplication> {
+        guard let bundle = Bundle(url: self.fileURL) else { return [] }
+        
+        var appExtensions: Set<ALTApplication> = []
+        
+        if let directory = bundle.builtInPlugInsURL, let enumerator = FileManager.default.enumerator(at: directory, includingPropertiesForKeys: nil, options: [.skipsSubdirectoryDescendants])
+        {
+            for case let fileURL as URL in enumerator where fileURL.pathExtension.lowercased() == "appex"
+            {
+                guard let appExtension = ALTApplication(fileURL: fileURL) else { continue }
+                appExtensions.insert(appExtension)
+            }
+        }
+        
+        return appExtensions
+    }
+}