Upgrade `_WP_List_Table_Compat` to PHP5-style constructor.

Add `public` to methods/members of `WP_Role`.
Add `public` to methods/members of `WP_User` where appropriate. Don't set `private` where indicated until more study has occurred and tests have been written for compatibiliy with existing magic methods.

See #27881, #22234.

Built from https://develop.svn.wordpress.org/trunk@28531


git-svn-id: http://core.svn.wordpress.org/trunk@28357 1a063a9b-81f0-0310-95a4-ce76da25c4cd