feat: Add BrowserWindow.setWindowButtonVisibility()

Fixes #12701 and supersedes #13131.

Ideally we would have added `setTitleBarStyle()`, but that is a
significantly more involved change. For example, consider the case where
we switch from `hidden` to `normal`. We would not only have to show the
traffic light buttons, we would also have to switch the window from a
frameless to a framed window and deal with various other window state.
Lets instead implement a simple solution for #12701.

atom/browser/native_window.h

@@ -187,6 +187,9 @@ class NativeWindow : public base::SupportsUserData,
   virtual void ToggleTabBar();
   virtual bool AddTabbedWindow(NativeWindow* window);
 
+  // Returns false if unsupported.
+  virtual bool SetWindowButtonVisibility(bool visible);
+
   // Toggle the menu bar.
   virtual void SetAutoHideMenuBar(bool auto_hide);
   virtual bool IsMenuBarAutoHide();