Add access modifiers to methods and members of list table classes:

* `WP_List_Table` is the base class that implements `__get()` and `__call()` for BC
* Adds unit tests to confirm that subclasses properly inherit magic methods
* Add modifiers to subclasses: `WP_Links_List_Table`, `WP_Media_List_Table`, `WP_MS_Sites_List_Table`, `WP_MS_Themes_List_Table`, `WP_MS_Users_List_Table`, `WP_Plugin_Install_List_Table`, `WP_Plugins_List_Table`, `WP_Posts_List_Table`, `WP_Terms_List_Table`, `WP_Theme_Install_List_Table`, `WP_Themes_List_Table`

See #27881, #22234.

Built from https://develop.svn.wordpress.org/trunk@28493


git-svn-id: http://core.svn.wordpress.org/trunk@28319 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add access modifiers to methods and members of `WP_Users_List_Table`.

See #27881, #22234.

Built from https://develop.svn.wordpress.org/trunk@28494


git-svn-id: http://core.svn.wordpress.org/trunk@28320 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add access modifier (`public`) to methods and members of `WP_Upgrader_Skin` and its subclasses.

See #27881, #22234.

Built from https://develop.svn.wordpress.org/trunk@28495


git-svn-id: http://core.svn.wordpress.org/trunk@28321 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add access modifier (`public`) to methods and members of `WP_Upgrader` and its subclasses.

See #27881, #22234.

Built from https://develop.svn.wordpress.org/trunk@28496


git-svn-id: http://core.svn.wordpress.org/trunk@28322 1a063a9b-81f0-0310-95a4-ce76da25c4cd