feat: implement BrowserWindow.moveTop on X11 (#16629)

It was implemented on Mac and Win but not on X11.
Tested on Ubuntu 16.04 and 18.04.

Also added a unit test in spec/api-browser-window-spec.js.
This test BrowserWindow.moveTop verifies that calling moveTop
on a window does not give the focus to this window.

notes: BrowserWindow.moveTop is now available on Linux/x11

https://github.com/electron/electron/issues/12516

atom/browser/native_window.h

@@ -113,9 +113,7 @@ class NativeWindow : public base::SupportsUserData,
   virtual double GetSheetOffsetX();
   virtual double GetSheetOffsetY();
   virtual void SetResizable(bool resizable) = 0;
-#if defined(OS_WIN) || defined(OS_MACOSX)
   virtual void MoveTop() = 0;
-#endif
   virtual bool IsResizable() = 0;
   virtual void SetMovable(bool movable) = 0;
   virtual bool IsMovable() = 0;