Make bus stop type mandatory

Tehdään bäkkärimuutokset omassa branchissa, kun ne voi tehdä täysin erillään
`currentAsset.propertyData` contains metadata about properties. If we use that to verify if property is required or not we can get rid of one extra copy of data (`currentAsset.requiredProperties`)

This would mean that we would assign `currentAsset.propertyData` also in `open` function, which would be a good thing since that would make the two modes (editing an existing bus stop and creating a new bus stop) identical.