Fix mismatches in access modifiers for `WP_List_Table` + subclasses.

Fixes #28843, #28879.

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4.0-beta1-29138.

Built from https://develop.svn.wordpress.org/trunk@29138


git-svn-id: http://core.svn.wordpress.org/trunk@28922 1a063a9b-81f0-0310-95a4-ce76da25c4cd