bug#37592: Using tabs in EWW

[Juri Linkov]

>> > Instead of adding another defcustom, how about doing this by default
>> > if tab-bar-mode is enabled?
>> 
>> I guess the users still need to decide whether they want to open
>> new tabs in the tab-bar (when eww-browse-url-new-window-is-tab is non-nil),
>> or new tabs in the tab-line (when eww-browse-url-new-window-is-tab is nil)
>> that just creates a new buffer displayed in the window-local tab-line -
>> this is the current behavior.
>> 
>> Another use case is that when tab-bar-mode is not yet enabled,
>> users migth prefer that clicking on a link with a Ctrl key
>> could enable the tab-bar and open a new tab in automatically
>> enabled tab-bar.
>
> It seems to me that there are sensible defaults in each of these
> situations, and we could use those defaults unconditionally until
> users complained.

Users complained, so I added that defcustom.