Add interface "Sortable" defining how a result set can be sorted

This interface should be used to ensure that sorting a result set
is standardized among the different query implementations.

library/Icinga/Data/Sortable.php

@@ -0,0 +1,48 @@
+<?php
+
+namespace Icinga\Data;
+
+/**
+ * Interface for sorting a result set
+ */
+interface Sortable
+{
+    /**
+     * Sort ascending
+     */
+    const SORT_ASC = 'ASC';
+
+    /**
+     * Sort descending
+     */
+    const SORT_DESC  = 'DESC';
+
+    /**
+     * Sort result set by the given field (and direction)
+     *
+     * Preferred usage:
+     * <code>
+     * $query->order('field, 'ASC')
+     * </code>
+     *
+     * @param  string   $field
+     * @param  int      $direction
+     *
+     * @return self
+     */
+    public function order($field, $direction = null);
+
+    /**
+     * Whether an order is set
+     *
+     * @return bool
+     */
+    public function hasOrder();
+
+    /**
+     * Get the order if any
+     *
+     * @return array|null
+     */
+    public function getOrder();
+}