* bookmark.el: Display the bookmark list header similarly to the

* bookmark.el: 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> with a few tweaks by me.

(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.