Improve customizability and better tab separators.

* lisp/tab-bar.el (tab-bar-tabs-function): New defvar.

* lisp/tab-line.el (tab-line-tab-name-function)
(tab-line-tabs-function): New defvars.
1 job for feature/tabs in 54 minutes and 11 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #3419
test-all

00:54:11