Fix page/section ID validation for cache files

[shaunix]

Mallard 1.0 does not allow you to easily swap out the type of the id attribute for page and section elements. Cache files need to change the type from NMTOKEN to something more permissive, like text, because sections in cache files are of the form page_id#section_id. The proposed solution for cache/1.0 is to just use new patterns cache_page_attr and cache_section_attr which are defined to look just like mal_page_attr and mal_section_attr, except the id change.

We should fix this for 1.1 so that cache/1.1 can just easily override the id attribute, and reuse mal_page_attr, mal_section_attr, and any extensions to those. If we introduce cache_page_attr and cache_section_attr, let's go ahead and keep them, though they may default to being empty. They could be useful extension points (e.g. for Sites).

If we let blocks take IDs and participate in linking, let's make sure we get this right the first time for them.

[shaunix]

Removing NeedsMEP label. I don't think we need a full write-up for a simple schema change. This is now in the 1.1 schema and spec, but I'm going to leave this issue open until there's a cache/1.1 schema that takes advantage of it.

[shaunix]

Cache Files 1.1 updated to use the new patterns.
http://projectmallard.org/cache/1.1/index.html