Page title fixes to accompany plugin fixes for CRM-12616

This PR is required by:
https://github.com/civicrm/civicrm-wordpress/pull/63

It sets the template title only in circumstances where the page title is needed - i.e. when 

* Civi is being displayed in WordPress admin
* Civi content is displayed via multiple shortcodes (whether within a single page/post or in an archive)

It also sets the global `$civicrm_wp_title` in all cases, so that it can be retrieved elsewhere once the Civi content has been rendered.