bug#37592: Using tabs in EWW

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Date: Wed, 02 Oct 2019 23:31:17 +0300
> 
> Like <C-mouse-1> in web browsers opens links in a new tab,
> the attached patch does the same for EWW.
> 
> The change that implements support for tabs in EWW is just 3 lines:
> 
>     (when eww-browse-url-new-window-is-tab
>       (let ((tab-bar-new-tab-choice t))
>         (tab-new)))
> 
> Everything else is replicating the existing code:
> 
> 1. A new defcustom eww-browse-url-new-window-is-tab is a copy
>    of existing browse-url-firefox-new-window-is-tab
> 
> 2. The docstring for eww-browse-url is copied from browse-url-firefox
> 
> 3. The code for shr-browse-url is copied from browse-url-at-point

Instead of adding another defcustom, how about doing this by default
if tab-bar-mode is enabled?