Data\Filter: add isChain() and isExpression()

This makes node type distiction easier

library/Icinga/Data/Filter/Filter.php

@@ -22,6 +22,10 @@ abstract class Filter
         return $this;
     }
 
+    abstract public function isExpression();
+
+    abstract public function isChain();
+
     abstract function toQueryString();
 
     public function getUrlParams()

library/Icinga/Data/Filter/FilterChain.php

@@ -166,6 +166,16 @@ abstract class FilterChain extends Filter
         }
     }
 
+    public function isExpression()
+    {
+        return false;
+    }
+
+    public function isChain()
+    {
+        return true;
+    }
+
     public function isEmpty()
     {
         return empty($this->filters);

library/Icinga/Data/Filter/FilterExpression.php

@@ -19,6 +19,16 @@ class FilterExpression extends Filter
         $this->expression = $expression;
     }
 
+    public function isExpression()
+    {
+        return true;
+    }
+
+    public function isChain()
+    {
+        return false;
+    }
+
     public function isEmpty()
     {
         return false;