MGCP: Translate text attributes

[mattjdnv]

Add support for translating attribute values where the user has stored the "text" version instead of the numeric one.

E.g. RST = "Hard/Paved" instead of RST=1

• This will probably double the size of the MGCP rules.
• Decide if the lookup should be stored statically, built on the fly or pulled from the schema as required.

[mattjdnv]

This ticket morphed into two parts: 1) "Thematic" MGCP layers 2) Handling "text" attributes

The second part has been put on hold due to the "text" attribute values in the datasets being from attributes that are not part of MGCP!