#91 Validation to require categorization

[Steve Pugh]

This patch make it possible to be able to require
authors to categorise under a given scheme. For more
info, see this thread:
http://sourceforge.net/forum/forum.php?thread_id=1558056&forum_id=368401

Each content type can be given "category contexts",
which are categorizations that are required by that
content type. For example, you could require Articles
to be categorized under LGNL, but PressReleases to be
categorized under RSS feeds.

In the "Content types" tab, the patch add a new field
"Category contexts". This is a comma-separated list of
"use contexts" which are required for each content
type. The default is empty, which means no vaidation is
performed.

Patch is attached. Please note, a new field is required
to "content_types" table, see SQL attachment.

[Steve Pugh]

patch

[Steve Pugh]

sql