Clean out layout columns API in WP_Screen.

* Move layout column setup into render_screen_meta() so that the number of columns is available earlier.
* Store the user provisioned number of columns in an instance var.
* Access the var with get_columns()
* Move all templates away from the screen_layout_columns global to the get_columns() method.
* Deprecate the global
* Remove the no longer needed check for 'auto' in the user option.
* Cast the user option to an int.

Props griffinjt
fixes #20506


git-svn-id: https://core.svn.wordpress.org/trunk@20579 1a063a9b-81f0-0310-95a4-ce76da25c4cd