UI changes that make things a bit cleaner

I didn't like the way that this plugin pushed a bunch of menu items directly into the "Edit" menu, so I've put them into a submenu and added a separator between the formatting items and the preference items. I've also centred the preferences window when it is shown. 

Feel free to reject or cherry pick what you want from this — I think a few wording and naming changes would make this plugin feel a lot more integrated with Xcode. I'll have a look at adding those tonight.