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/ui/x/x_window_utils.h

@@ -23,6 +23,9 @@ void SetWindowType(::Window xwindow, const std::string& type);
 // Returns true if the bus name "com.canonical.AppMenu.Registrar" is available.
 bool ShouldUseGlobalMenuBar();
 
+// Bring the given window to the front and give it the focus.
+void MoveWindowToForeground(::Window xwindow);
+
 }  // namespace atom
 
 #endif  // ATOM_BROWSER_UI_X_X_WINDOW_UTILS_H_