added message contents to the messages

addedn the conentents for all messages regarding the BPMN diagramm and own interpretation.
also created an identifier for pieces to be used for network communication between server and client so that they talk about the same piece.

Projekte/mdga/model/src/main/java/pp.mdga/game/PlayerData.java

@@ -19,7 +19,7 @@ public PlayerData(Color color) {
         waitingArea = new Piece[4];
         for (int i = 0; i < 4; i++) {
             homeNodes[i] = new HomeNode();
-            pieces[i] = new Piece(color, PieceState.WAITING);
+            pieces[i] = new Piece(color, PieceState.WAITING, i);
             waitingArea[i] = pieces[i];
         }
     }