[BUG] Show import/export panel as modal panel.

This approach would ensure the panel show in most front and in center of screen.

ShadowsocksX-NG/ServerProfileManager.swift

@@ -79,7 +79,7 @@ class ServerProfileManager: NSObject {
         openPanel.canCreateDirectories = false
         openPanel.canChooseFiles = true
         openPanel.becomeKey()
-        openPanel.begin { (result) -> Void in
+        let result = openPanel.runModal()
         if (result == NSFileHandlingPanelOKButton && (openPanel.url) != nil) {
             let fileManager = FileManager.default
             let filePath:String = (openPanel.url?.path)!
@@ -124,7 +124,6 @@ class ServerProfileManager: NSObject {
             }
         }
     }
-    }
     
     func exportConfigFile() {
         //读取example文件，删掉configs里面的配置，再用NSDictionary填充到configs里面
@@ -164,7 +163,7 @@ class ServerProfileManager: NSObject {
         savePanel.allowedFileTypes = ["json"]
         savePanel.nameFieldStringValue = "export.json"
         savePanel.becomeKey()
-        savePanel.begin { (result) -> Void in
+        let result = savePanel.runModal()
         if (result == NSFileHandlingPanelOKButton && (savePanel.url) != nil) {
             //write jsonArr1 back to file
             try! jsonString.write(toFile: (savePanel.url?.path)!, atomically: true, encoding: String.Encoding.utf8)
@@ -176,7 +175,6 @@ class ServerProfileManager: NSObject {
                 .deliver(notification)
         }
     }
-    }
     
     class func showExampleConfigFile() {
         //copy file to ~/Downloads folder