translations.txt: specify which translation takes precedence over another

[jsteelz]

Currently, it is possible to define translations in two ways using translations.txt:

1. By referring to a particular row using record_id and record_sub_id;
2. By referring to a particular value using field_value;

However the spec currently does not forbid doing the following:

routes.txt

route_id,route_short_name
route_id_1,subway

translations.txt

table_name,field_name,language,translation,record_id,record_sub_id,field_value
routes,route_short_name,fr,métro,route_id_1,,
routes,route_short_name,fr,train,,,subway

In which case there is no stated logic over which translation should take preference. This logic should be defined clearly - I propose that a specific reference to a record using record_id and record_sub_id should be used over the more 'generic' case that using field_value represents.

[timMillet]

PR #327 replaces this issue