put value in defvar

The previous idiom of (defvar foo nil) (if foo () (setq foo foo-value))
is not needed because defvar doesn't change value of existing variables.

Doing eval-defun (C-M-x) on a defvar does update value which is convenient.
use ` and regexp-opt in txl-mode-keywords definition
font-lock-constant-face replaces obsolete font-lock-reference-face
don't include newline in highlighting ' quoted tokens
' escapes comments, _ is word separator for movement
' doesn't escape inside strings
rewrite txl-mode-get-name so it works
last file used for run/debug is default for next time