Adding a key/value model

This is meant to represent the different hash maps coming from plain
mapserver map objects (metadata, format options ...)
mapsettings - ogc options

Using the previously created model for ogc options lists.
mapsettings - removing metadatas on OGC tab
mapsettings - output format options

- Finishing refactor to use the (simpler) KeyValue model
- ui cosmetics to use the maximum room available in tab