OGR provider: deal with wkbUnknown25D (wkbUnknown | wkb25DBit)

This geometry type doesn't exist admitedly in the OGRwkbGeometryType
enumeration but a few OGR drivers can expose layers of this type.
Those layers should be dealt exactly like wkbUnknown layers, i.e.
QGIS should analyze the content to see if there are points, lines,
polygons and create derived layers.

Also report the geometry type of wkbGeometryCollection25D layers in
the U.I.
Convert expressions to target field format