Add 'yyyy-MM-dd' to the allowed formats for the StringToDateTimeConverter

This is needed for the sql date type.
In my case, I have a type table with a column of the type 'date'.

Some problems that occured:
Because the framework does not parse 'yyyy-MM-dd'...
1) date values on xforms are empty 
2) the FormatDate scriptfunction can't handle this type of date.
3) ...