Introduce show_in_menu for register_taxonomy.

Accepts boolean: true to show, false to hide. If not set, the default is inherited from show_ui.

fixes #20930.
Built from https://develop.svn.wordpress.org/trunk@25133


git-svn-id: http://core.svn.wordpress.org/trunk@25113 1a063a9b-81f0-0310-95a4-ce76da25c4cd