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/api/atom_api_top_level_window.h

@@ -126,9 +126,7 @@ class TopLevelWindow : public mate::TrackableObject<TopLevelWindow>,
   void SetResizable(bool resizable);
   bool IsResizable();
   void SetMovable(bool movable);
-#if defined(OS_WIN) || defined(OS_MACOSX)
   void MoveTop();
-#endif
   bool IsMovable();
   void SetMinimizable(bool minimizable);
   bool IsMinimizable();