The significance of a MetadataKey to a user.
| mkt_significant | A key of significant interest, to be shown early on. |
| mkt_normal | A normal key, to be shown underneath mkt_significant keys. |
| mkt_author | Should only be shown if the user asks for author information. |
| mkt_dependencies | Should only be shown if the user asks for dependencies. |
| mkt_internal | Should not usually be displayed to the user. |
| last_mkt | Number of keys in paludis::MetadataKeyType. |
1.5.4