add label icons back-end (model and database)

wise-webapp/src/main/java/com/wisemapping/rest/model/RestLabel.java

@@ -65,10 +65,18 @@ public class RestLabel {
         label.setColor(color);
     }
 
+    public void setIcon(@NotNull final String icon) {
+        label.setIcon(icon);
+    }
+
     @Nullable public String getColor() {
         return label.getColor();
     }
 
+    @Nullable public String getIcon() {
+        return label.getIcon();
+    }
+
     @JsonIgnore
     public Label getDelegated() {
         return label;