Read mission dialogue as translation objects

Rather than a simple string, read these entries as translation objects.
This has the advantage that they are checked for correct text styling.

src/mission.cpp

@@ -673,7 +673,7 @@ std::string mission::dialogue_for_topic( const std::string &in_topic ) const
 
     const auto &response = type->dialogue.find( topic );
     if( response != type->dialogue.end() ) {
-        return _( response->second );
+        return response->second.translated();
     }
 
     return string_format( "Someone forgot to code this message id is %s, topic is %s!",

src/mission.h

@@ -241,7 +241,7 @@ struct mission_type {
         std::function<void( mission * )> end = mission_end::standard;
         std::function<void( mission * )> fail = mission_fail::standard;
 
-        std::map<std::string, std::string> dialogue;
+        std::map<std::string, translation> dialogue;
 
         // A dynamic goal condition invoked by MGOAL_CONDITION.
         std::function<bool( const mission_goal_condition_context & )> goal_condition;