Add support for 256-color and 24bit ANSI colors in term-mode

Add support for 256-color and 24bit ANSI colors in term-mode

(term-ansi-face-already-done): Make obsolete
(term--maybe-brighten-color): Remove
(term--color-as-hex): New function
(term-handle-colors-array): Make obsolete in favour of the new
function 'term--handle-colors-list'.
(term--handle-colors-list): New function, that can also handle ANSI
codes 38 and 48.
(term-handle-ansi-escape): Use it

* test/lisp/term-tests.el (ansi-test-strings): Add tests for 256-color
and 24bit ANSI colors