Deprecate get_current_theme(). Use (string) wp_get_theme() to get the...

Deprecate get_current_theme(). Use (string) wp_get_theme() to get the translated name of the theme. Keep the current_theme option for now. see #20103, see #20138.

git-svn-id: https://core.svn.wordpress.org/trunk@20040 1a063a9b-81f0-0310-95a4-ce76da25c4cd