General improvements to NS port

General improvements to NS port

* src/dispextern.h: Remove some !HAVE_NS conditionals around grab
related code.
* src/frame.c (gui_mouse_grabbed, gui_redo_mouse_highlight): Remove
!HAVE_NS conditionals around code.

* src/nsmenu.m (ns_update_menubar): Prevent recursive calls and enable
shallow updates on GNUstep.
(menuNeedsUpdate): Prevent recursive calls.
(ns_menu_show): Fix mysterious GC-related bug.
(update_frame_tool_bar_1): Work around mysterious toolbar sizing bug
on GNUstep.

* src/nsterm.h (struct ns_output): New field for tracking toolbar
visibility changes.

* src/nsterm.m (frame_set_mouse_pixel_position): Implement for
GNUstep.
(ns_redraw_scroll_bars): Enable for GNUstep.
(ns_clear_frame): Redraw scrollbars on GNUstep.
(ns_update_window_end): New function.
(ns_redisplay_interface): Add ns_update_window_end on GNUstep.
(- keyDown): Remove debug code that doesn't work on GNUstep.
(- mouseDown): Enable grab tracking on NS port.
(- resizeWithOldSuperviewSize): Fix build with NSTRACE.

* src/xdisp.c (note_tab_bar_highlight): Enable some code for NS port.