Break dependency on calendar.el (i.e. do not modify calendar-mode-map),

Break dependency on calendar.el (i.e. do not modify calendar-mode-map),
use easy-menu, and make sure that C-h k can be used on the menu entries.
(cal-menu-holiday-window-suffix, cal-menu-set-date-title): New funs.
(cal-menu-moon-menu, cal-menu-diary-menu, cal-menu-holidays-menu)
(cal-menu-goto-menu, cal-menu-scroll-menu): New consts.
(cal-menu-context-mouse-menu, cal-menu-global-mouse-menu): New menus.
(calendar-flatten, cal-menu-update): Remove.
(calendar-mouse-insert-hebrew-diary-entry)
(calendar-mouse-insert-islamic-diary-entry)
(calendar-mouse-insert-bahai-diary-entry):
Remove (fold into cal-menu-diary-menu).
(calendar-mouse-2-date-menu, calendar-mouse-cal-tex-menu)
(cal-tex-mouse-filofax): Remove (fold into cal-menu-context-mouse-menu).
(calendar-mouse-3-map): Remove (turn into cal-menu-global-mouse-menu).
(calendar-mouse-view-diary-entries): Minor simplifications.
(calendar-event-to-date): Use with-current-buffer.