Add a `$run_texturize` static var to `wptexturize()` that is filterable via a new run-once filter: `'run_wptexturize'`. Allows user to disable texturization.

Needs filter docs.

Props nacin, SergeyBiryukov.
See #19550.

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git-svn-id: http://core.svn.wordpress.org/trunk@28529 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Ensure that `shortcode_unautop()` treats `&nbsp;` like whitespace.
`shortcode_unautop()` and `wptexturize()` now use `wp_spaces_regexp()` instead of raw regex.

Adds unit tests.

Props miqrogroove.
See #27588.

Built from https://develop.svn.wordpress.org/trunk@28716


git-svn-id: http://core.svn.wordpress.org/trunk@28530 1a063a9b-81f0-0310-95a4-ce76da25c4cd