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

[Liang-Jie Lee]

As title. I think this might be useful for people who disable the external
border and use tab-bar as their status bar. I found there is option to
move the frame by draging header-line (the drag-with-header-line frame
parameter), so I think it's reasonable to also support something like
"drag-with-tab-bar-line" for tab-bar-mode users.



In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo 
version 1.16.0)
 of 2021-06-21 built on debian
Repository revision: 09f17ac4752e18bf834d2f20ceef561cc516d917
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux 11 (bullseye)

Configured using:
 'configure --with-mailutils --with-native-compilation --with-xwidgets
 --enable-link-time-optimization --with-xdbe=no 'CFLAGS=-march=native
 -O3''

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF
TOOLKIT_SCROLL_BARS X11 XIM XPM XWIDGETS GTK3 ZLIB

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Load-path shadows:
/home/booaa/.emacs.d/elpa/transient-20210619.1100/transient hides 
/usr/local/share/emacs/28.0.50/lisp/transient

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Memory information:
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