whitespace: Redraw if indentation or line length changes

whitespace: Redraw if indentation or line length changes

* lisp/whitespace.el (whitespace-color-on): Convert the indentation
matcher from a static regular expression to a function so that changes
to `indent-tabs-mode' and `tab-width' are picked up the next time
`font-lock-flush' runs.
(whitespace--indentation-matcher): The new function matcher.
(whitespace--variable-watcher): New variable watcher that calls
`font-lock-flush' if `whitespace-mode' is enabled for the buffer.
(whitespace--watched-vars): List of variables to watch.
(whitespace-unload-function): Un-watch the variables. (bug#56103).