fill out a %this variable for trigger callbacks

Engine/source/T3D/trigger.cpp

@@ -731,7 +731,8 @@ void Trigger::potentialEnterObject(GameBase* enter)
 
       if(evalCmD(&mEnterCommand))
       {
-         String command = String("%obj = ") + enter->getIdString() + ";" + mEnterCommand;
+         String command = String("%obj = ") + enter->getIdString() + ";";
+         command = command + String("%this = ") + getIdString() + ";" + mEnterCommand;
          Con::evaluate(command.c_str());
       }
 
@@ -779,7 +780,8 @@ void Trigger::processTick(const Move* move)
             
             if (evalCmD(&mLeaveCommand))
             {
-               String command = String("%obj = ") + remove->getIdString() + ";" + mLeaveCommand;
+               String command = String("%obj = ") + remove->getIdString() + ";";
+               command = command + String("%this = ") + getIdString() + ";" + mLeaveCommand;
                Con::evaluate(command.c_str());
             }
             if (testTrippable() && testCondition())