replace duplicate page type icon with a 3-column one

There are 5 built-in choices for page type icons when adding new pagetypes (or editing the "settings" of an existing page type), but 2 of the 5 icons are exactly the same. This replaces one of them with another useful icon you could use for a 3-column layout.

The only drawback here is people who chose the 2nd of the 2 "right sidebar" icons will now all of a sudden see the 3-column icon. Pretty easy to fix by editing the page type settings in the dashboard, but I could see how this might not be ideal. A more complex solution would be to remove that duplicate icon entirely and have some hacky code that does a "backwards compatibility check" and serves up the old icon when needed -- but that doesn't sound like a lot of fun to implement or maintain.