bug#42052: 28.0.50; tab-bar-mode should be frame-local

[James N. V. Cash]

When setting tab-bar-show to `1`, I expect the tab bar to only show if
there is more than one tab in the frame.

This behaves as expected when there is only one frame, but if I have
multiple frames open, then if any one of them has more than one tab, it
turns on tab-bar-mode, which then makes any other frame show the tab
bar, even if they only have one tab in them.

Would it be possible to have tab-bar-mode showing be frame-local?

In GNU Emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.30, cairo 
version 1.15.10)
 of 2020-06-20 built on gonk
Repository revision: 3af631dcf28a5964d9e56c9be8ee7f2125d90d8a
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Ubuntu 18.04.4 LTS

Configured using:
 'configure --with-xwidgets --with-cairo'

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