The attached patch changes bookmark-bmenu-mode to be based on
tabulated-list-mode instead of special-mode.

This allows us to simplify the code in several cases.  In addition, we
get many features for free, such as sorting columns by clicking on the
column headers, changing size of columns.  In the future, this will
obviously include any new feature added to 'tabulated-list-mode'.

The only functional step backwards is that we no longer support the
optional "inline" header line -- a bookmark.el-specific hack to have a
header without using 'header-line-format'.  I don't believe this
feature is very useful since the lack of such support for anything
similar in e.g. 'package-menu-mode' has not caused any problems.  It
seems to have been added together with 'header-line-format' as a fire
escape if the latter caused any problems.

I recently added a number of tests to bookmark.el on master, which
were developed as part of this suggested change.  These tests pass
using both the new and the old code, which gives some degree of
confidence in this change.

Any comments, objections or suggestions?  Thanks.

Best regards,
Stefan Kangas