Expose ReadOnlyValueMapper to be set

[andrewmckaskill]

I have quite a number of fields that are drop down lists, with values and captions populated from a different database table.

In read-only mode by default they use the "Label" property editor and just show the text version of the value.

I can't use the format function because I can't inject my database connection and associated services (the DI container hasn't been built yet).

Could you make the ReadOnlyValueMapper public instead of internal (with the appropriate method to set it) so that I can inject a ValueMapper that gets the nice caption from the database.