Filter: allow to list all filtered columns

This is important for backends that need to know which additional columns
to fetch.

refs #4787

library/Icinga/Data/Filter/Filter.php

@@ -52,6 +52,8 @@ abstract class Filter
         return false === strpos($this->id, '-');
     }
 
+    abstract public function listFilteredColumns();
+
     public function applyChanges($changes)
     {
         $filter = $this;

library/Icinga/Data/Filter/FilterChain.php

@@ -117,6 +117,20 @@ abstract class FilterChain extends Filter
         return $this->operatorSymbol;
     }
 
+    public function listFilteredColumns()
+    {
+        $columns = array();
+        foreach ($this->filters as $filter) {
+            if ($filter instanceof FilterExpression) {
+                $columns[] = $filter->getColumn();
+            } else {
+                $columns += $filter->listFilteredColumns();
+            }
+        }
+        // array_unique?
+        return $columns;
+    }
+
     public function toQueryString()
     {
         $parts = array();

library/Icinga/Data/Filter/FilterExpression.php

@@ -59,6 +59,11 @@ class FilterExpression extends Filter
         return $this;
     }
 
+    public function listFilteredColumns()
+    {
+        return array($this->getColumn());
+    }
+
     public function __toString()
     {
         $expression = is_array($this->expression) ?