Use common memory management functions for lwlib and refactor users.

Use common memory management functions for lwlib and refactor users.
* lwlib/lwlib.h (widget_value): Do not maintain a free list any more.
(malloc_widget_value, free_widget_value): Remove prototypes.
* lwlib/lwlib.c (malloc_widget_value, free_widget_value):
(widget_value_free_list, malloc_cpt): Remove.
(free_widget_value_tree, copy_widget_value_tree): Adjust users.
* src/menu.h (xmalloc_widget_value): Replaced by ...
(make_widget_value): ... new prototype.
* src/menu.c (xmalloc_widget_value): Replaced by ...
(make_widget_value): ... new function.
(free_menubar_widget_value_tree, digest_single_submenu): Adjust users.
* src/gtkutil.c (malloc_widget_value, free_widget_value):
(widget_value_free_list, malloc_cpt): Remove old lwlib-compatible code.
* src/keyboard.h (enum button_type, struct _widget_value):
* src/gtkutil.h, src/nsgui.h, src/w32gui.h (malloc_widget_value):
(free_widget_value): Likewise.
* src/nsmenu.m (ns_update_menubar, ns_menu_show):
* src/w32menu.c (set_frame_menubar, w32_menu_show, w32_dialog_show):
* src/xmenu.c (set_frame_menubar, xmenu_show, x_dialog_show): Adjust users.
* src/xterm.h (XtParent) [USE_GTK]: Remove unused macro.