A few touch-ups for the + icon.

1. When we generate the top level headline for the year, instead of setting the type to calendar, we set the icon to calendar.

2. If there's already a type of calendar on the year headline, we remove it. Any other type, we leave alone.

3. When creating a headline, the type goes on the headline we create, not on its parent. This way when we generate a web index for a calendar structure, the document boundary will be where users expect it.