Add "New Tab" button for Native macOS Tabs

Adds responders for `newWindowForTab` to `AtomApplicationDelegate` and
`NativeWindowMac`, so that `BrowserWindow`s with a `tabbingIdentifier`
will get the new tab button, and both `app` and `window` will emit a
`new-tab-for-window` event.

docs/api/app.md

@@ -149,6 +149,16 @@ ID as the activity's source app and that supports the activity's type.
 Supported activity types are specified in the app's `Info.plist` under the
 `NSUserActivityTypes` key.
 
+### Event: 'new-window-for-tab' _macOS_
+
+Returns:
+
+* `event` Event
+
+Emitted when the user clicks the native macOS new tab button. The new
+tab button is only visible if the current `BrowserWindow` has a
+`tabbingIdentifier`
+
 ### Event: 'browser-window-blur'
 
 Returns:

docs/api/browser-window.md

@@ -239,7 +239,9 @@ It creates a new `BrowserWindow` with native properties as set by the `options`.
     `maximize()` directly. Default is `false`.
   * `tabbingIdentifier` String (optional) - Tab group name, allows opening the
     window as a native tab on macOS 10.12+. Windows with the same tabbing
-    identifier will be grouped together.
+    identifier will be grouped together. This also adds a native new tab button
+    to your window's tab bar and allows your `app` and window to receive the
+    `new-window-for-tab` event.
   * `webPreferences` Object (optional) - Settings of web page's features.
     * `devTools` Boolean (optional) - Whether to enable DevTools. If it is set to `false`, can not use `BrowserWindow.webContents.openDevTools()` to open DevTools. Default is `true`.
     * `nodeIntegration` Boolean (optional) - Whether node integration is enabled. Default
@@ -547,6 +549,10 @@ Emitted when the window opens a sheet.
 
 Emitted when the window has closed a sheet.
 
+#### Event: 'new-window-for-tab' _macOS_
+
+Emitted when the native new tab button is clicked.
+
 ### Static Methods
 
 The `BrowserWindow` class has the following static methods: