bug#49247: 28.0.50; [Feature Request] Make tab-bar-lines dragable

[Eli Zaretskii]

> Cc: Eli Zaretskii <eliz@gnu.org>, 49247@debbugs.gnu.org,
>  s930054123yaoyao@gmail.com
> From: martin rudalics <rudalics@gmx.at>
> Date: Thu, 1 Jul 2021 09:54:49 +0200
> 
> It should be also possible to drag a frame with the tab bar.

That would require significant changes in how mouse clicks on the tab
bar are processed.  Currently, if the click is not on some glyph of
the tab-bar text, it is ignored.