Fix a bug where the wrong menu would be triggered by mouse

Fix a bug where the wrong menu would be triggered by mouse

For layouts such as the following, clicking the "l" in Tools with the
right window focused would trigger the File menu, not the Tools menu.
This is because the event would have window coordinate (1 . 0).
Similarly, clicking the "p" in Help would trigger the Edit menu.

Example Emacs frame:
+--------------------------------------------------------+
|File Edit Options Buffers Tools Help                    |
|;; This buffer is for text$|;; This buffer is for text $|
|;; To create a file, visit$|;; To create a file, visit $|
|                           |                            |
|                           |                            |
|-UUU:----F1  *scratch*     |-UUU:----F1  *scratch*      |
|                                                        |
+--------------------------------------------------------+
* lisp/menu-bar.el (menu-bar-open-mouse): Reject clicks not on
the menu bar.
*lisp/xt-mouse.el (xterm-mouse-event): Pass the current frame to
'posn-at-x-y', to make the effect consistent with other mouse-handling
features.