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.

atom/browser/native_window_mac.mm

@@ -7,6 +7,7 @@
 #include <Quartz/Quartz.h>
 #include <string>
 
+#include "atom/browser/browser.h"
 #include "atom/browser/native_browser_view_mac.h"
 #include "atom/browser/ui/cocoa/atom_touch_bar.h"
 #include "atom/browser/window_list.h"
@@ -417,6 +418,11 @@ bool ScopedDisableResize::disable_resize_ = false;
   shell_->NotifyWindowSheetEnd();
 }
 
+- (IBAction)newWindowForTab:(id)sender {
+  shell_->NotifyNewWindowForTab();
+  atom::Browser::Get()->NewWindowForTab();
+}
+
 @end
 
 @interface AtomPreviewItem : NSObject <QLPreviewItem>