[[[
Display the bookmark list header similarly to the buffer list header
(see `list-buffers'), where the default is now an immovable/immutable
header line.

Patch by: Matthias Meulien <orontee {_AT_} gmail.com>
          Karl Fogel <kfogel {_AT_} red-bean.com>

* lisp/bookmark.el
  (bookmark-bmenu-use-header-line): New variable.
  (bookmark-bmenu-inline-header-height): New name for
    `bookmark-bmenu-header-height', to avoid confusion with the code
     for the new immovable header.  All references changed.
  (bookmark-bmenu-set-header): New function.
  (bookmark-bmenu-list, bookmark-bmenu-toggle-filenames):
    Conditionalize header construction accordingly.
  (bookmark-bmenu-ensure-position): Conditionalize the skipping of the
    inline header height.
  (bookmark-bmenu-show-filenames, bookmark-bmenu-hide-filenames): Conditionalize
    the skipping of the inline header height.
]]]