[Accessibility] Add WAI-ARIA landmarks to the Prosilver template files

The WAI-ARIA landmark roles are a way to provide more navigational points on a page for users who are blind and use a screen reader. With these landmark roles, modern screen readers can quickly jump to the banner (AKA header), navigation, search, main, complementary and contentinfo (footer) parts of a page. Even multiple landmarks for navigation/search etc. are possible. A full list of landmark roles can be found here:
http://www.w3.org/TR/wai-aria/#roleattribute_inherits

The attached patch applies those landmark roles to the current SVN trunk of phpBB 3.1. Thanks to the good design of the Prosilver template, the landmarks actually needed to be applied to a few files only. They have no impact on the layout of a page and are pure semantic information that screen readers use.