Added a key property, so specific views can easily be referenced.

Author: Simon Brown

structurizr-core/src/com/structurizr/view/View.java

@@ -13,6 +13,7 @@ public abstract class View implements Comparable<View> {
     private SoftwareSystem softwareSystem;
     private String softwareSystemId;
     private String description = "";
+    private String key;
     private PaperSize paperSize = PaperSize.A4_Portrait;
 
     private Set<ElementView> elementViews = new LinkedHashSet<>();
@@ -67,6 +68,14 @@ public void setDescription(String description) {
         }
     }
 
+    public String getKey() {
+        return key;
+    }
+
+    public void setKey(String key) {
+        this.key = key;
+    }
+
     public PaperSize getPaperSize() {
         return paperSize;
     }