Deprecate WP_TEMPLATE_PATH

This solves https://github.com/woothemes/woocommerce/issues/5483

Also the WordPress core guys have deprecated STYLESHEETPATH and TEMPLATEPATH (https://core.trac.wordpress.org/changeset/28563) (for version 4.0). 

It will be great to deprecate WP_TEMPLATE_PATH too, basically for the same reason as TEMPLATEPATH and TEMPLATEPATH and a few other reasons.