WP_Theme should use get_file_data() for retrieving page templates

[dd32]

Currently WP_Theme::get_page_templates() uses a direct regular expression on the result of get_file_contents(), it should instead use the get_file_data() helper method we have to interact with file header data.

Currently switching will result in a back-compat break, as some themes have been using headers such as <?php // Template Name: Something ?> as the header, which get_file_data() doesn't like - See #33387

See r21117 for where we previously did use it temporarily during the 3.4 cycle, but broke due to #33387

[m_uysl]

depends on #33387

[Thomas Vitale]

Patch refreshed