[feature] value constraints for known fields

[KS-HTK]

It is possible to set value constraints for attributes on QGis layers.

---

For bus layers

name	type	values	features
pp_type	QString	"bus" or "line"
pp_index	int		enforce unique (for bus layers only if possible)
in_service	double	1 or 0

---

for line layers

name	type	values	features
pp_type	QString	"bus" or "line"
pp_index	int		enforce unique (for line layers only if possible)
in_service	double	1 or 0

possibly limit std_type to a known string if metadata can be added to qgis