WP_Debug_Data: Extract wp-themes data into separate methods.

This is the last part in a larger modularization of the data in `WP_Debug_Data`. Previously this was a single massive method drawing in debug data from various groups of related data, where the groups were independent from each other.

This patch separates the findal set of twelve groups, the `wp-active-theme`, `wp-parent-theme`, and `wp-themes-inactive` info, into a separate methods focused on those data.

This work precedes changes to make the `WP_Debug_Data` class more extensible for better use by plugin and theme code.

Developed in WordPress#7507
Discussed in https://core.trac.wordpress.org/ticket/61648

Props apermo, dmsnell.
Fixes #61648.


git-svn-id: https://develop.svn.wordpress.org/trunk@59176 602fd350-edb4-49c9-b593-d223f7449a82

Author: dmsnell