Incorrect page title when embedding the privatemsg_new form

As noted in #705804: Incorrect page title → and 🐛 Embedding private message - causes the page title to be over written Closed: outdated , privatemsg_new hardcodes the page title so when the form is embedded somewhere else, page title of that page is changed as well.

Attached is the patch - before changing the title it makes sure that the user is on "messages/new" page.

Sidenote: I see the message about 7.x-2.x-dev, but thought it might be useful for others if I post the patch for this branch as well.

Let me know if any comments - thanks!