Uses UTIs to determine whether apps are installed or not

AltStore now inserts an app-specific UTI when resigning apps, and it periodically checks whether that app has been deleted by checking whether UTTypeCopyDeclaration returns nil for the same app-specific UTI.

AltStore/Model/InstalledApp.swift

@@ -187,6 +187,12 @@ extension InstalledApp
         return directoryURL
     }
     
+    class func installedAppUTI(forBundleIdentifier bundleIdentifier: String) -> String
+    {
+        let installedAppUTI = "io.altstore.Installed." + bundleIdentifier
+        return installedAppUTI
+    }
+    
     var directoryURL: URL {
         return InstalledApp.directoryURL(for: self)
     }
@@ -198,4 +204,8 @@ extension InstalledApp
     var refreshedIPAURL: URL {
         return InstalledApp.refreshedIPAURL(for: self)
     }
+    
+    var installedAppUTI: String {
+        return InstalledApp.installedAppUTI(forBundleIdentifier: self.resignedBundleIdentifier)
+    }
 }