[emacs-w3m:13537] Tabs

Hi,

The native tabs feature is being developed in the "feature/tabs"
branch of GNU Emacs now.  There are tab-bar and tab-line.
tab-bar is an analogy with tool-bar or menu-bar, and tab-line is
an analogy with header-line or mode-line.  See the thread that
begins with this article posted to the emacs-devel list:

<https://lists.gnu.org/archive/html/emacs-devel/2019-08/msg00658.html>

There Juri Linkov wrote:
> Authors of all packages that currently had no choice other than to
> misuse the header-line for displaying window tabs are welcome now to
> adapt their packages for displaying window tabs on the new dedicated
> tab-line that doesn't conflict with the header-line anymore.

Though I don't think the use of header-line in emacs-w3m is
necessarily misuse, it is certain that other Emacs applications
using header-line will conflict with emacs-w3m[1].  So, I tried
making emacs-w3m use tab-line instead of header-line.  It was
quit easy.  What I did was performing 's/header-line/tab-line/'
on w3m-ems.el, w3m-search.el, w3m-util.el, and w3m.el.  So far I
have no idea how to switch header-line and tab-line according to
whether the new tabs feature is available or not, though.

Note: Emacs to use should be built from the branch of course.

[1] So, how do we solve the conflict with emacs-w3m and other
applications that use tab-line and run together with emacs-w3m?