Allow users to select the header tag for widget titles

Relatively recently, the ability to chose the header tag was added to Panels/CTools. However, it didn't really fit in with the way we layout/restructure the widget settings form, so we hid it in panopoly_magic. But this would be a super useful feature!

I propose that we add a checkbox labeled "Change title header type" which goes right below the title override textfield. When this checkbox is checked, we show the select for the changing the header tag. This way, we keep it out of the way for the majority of users who don't care or understand what this is about, but still allow it for advanced use cases!