Allow sorting relation on arbitrary keys

[jidanni]

https://github.com/MarcusWolschon/osmeditor4android/issues/1972#issuecomment-1337599445_ says make new issue:

Allow sorting relations on are arbitrary keys. E.g., numbered trail loop markers. ref=01 ref=02 ref=03... Note the user must first be able to see arbitrary keys without needing to click each one, before being allowed to sort on them, otherwise he might sort into some mess that he cannot then see.

Let's see, for our trail markers we might want to sort stringwise on the ref field.

For highway, and railroad mileposts, we might want to sort numerically, on the https://wiki.openstreetmap.org/wiki/Key:distance fields.

So we see all sort methods, and all fields, should be available to choose from.