Fix GTK3 menubar items not having a border.

GTK3 menubar items were accidentally set to border-width: 0 instead of
1px. This fixes them and improves the overall look.

OLD:
![](http://i.imgur.com/d3O8Lz1.png)

NEW:
![](http://i.imgur.com/gyHmeiT.png)

It's subtle, but important and greatly improves the overall look - it can be altered quite dramatically by other shadings as well.