Add keywords support to Texture

[michael]

Group says:

• we would only need one keyword group
• we could add the optional content-type attribute for kwd elements to group keywords
• Users would prefer keyword sections in the user interface
• We would need one additional keyword group per translated language

[michael]

Spec proposal:

<kwd-group>
  <kwd content-type="author-keyword">optogenetics</kwd>
  <kwd content-type="author-keyword">two-photon</kwd>
  <kwd content-type="author-keyword">calcium imaging</kwd>
  <kwd content-type="research-organism">Mouse</kwd>
</kwd-group>

This model is equivalent to typed tags. We keep everything in a flat structure but allow the keyword to be member of a group. Using this model we can make Texture configurable to support journal- and subject-specific keywords.

[michael]

First version done, including category property and multiple languages.