Add an attribute "Description" on Category meta model

[cgourdin]

To allow edition of documentation for categories (kind, mixin etc.) when designing an extension.

This field will be used on documentation generation (textile, latex) and will be rendered with model@runtime on model description output. To edit description, this feature will use a memo (multi-line editor).

[philippemerle]

I agree to add a description attribute to categories in order to document detailed description. The Catefory.tilte attribute is more like a short description.