add function isajaxrequest() (#1400)

* Update Taxonomy.php

* Update TwigExtension.php

* Update TwigExtension.php

Incorrect comment updated.

* Update TwigExtension.php

Add function ishuman() to detect agent disposition.

* Update TwigExtension.php

* Revert "Merge remote-tracking branch 'origin/develop' into develop"

This reverts commit 7a59a84, reversing
changes made to 102973d.

* Revert "Revert "Merge remote-tracking branch 'origin/develop' into develop""

This reverts commit 50fc775.

* Attempt to resolve conflicts

* param comment corrected

* Update TwigExtension.php

* Update TwigExtension.php

* Update TwigExtension.php

* Update TwigExtension.php

* add isajaxrequest

Check HTTP_X_REQUESTED_WITH header. Non critical use only. Handy for
streamlining ajax presentation.

* remove ishuman

* remove Browser class

system/src/Grav/Common/Twig/TwigExtension.php

@@ -119,6 +119,7 @@ public function getFunctions()
             new \Twig_SimpleFunction('get_cookie', [$this, 'getCookie']),
             new \Twig_SimpleFunction('redirect_me', [$this, 'redirectFunc']),
             new \Twig_SimpleFunction('range', [$this, 'rangeFunc']),
+            new \Twig_SimpleFunction('isajaxrequest', [$this, 'isAjaxFunc']),
         ];
     }
 
@@ -892,4 +893,17 @@ public function rangeFunc($start = 0, $end = 100, $step = 1)
     {
         return range($start, $end, $step);
     }
+
+    /**
+     * Check if HTTP_X_REQUESTED_WITH has been set to xmlhttprequest,
+     * in which case we may unsafely assume ajax. Non critical use only.
+     *
+     * @return true if HTTP_X_REQUESTED_WITH exists and has been set to xmlhttprequest
+     */
+    public function isAjaxFunc()
+    {
+        return (
+            !empty($_SERVER['HTTP_X_REQUESTED_WITH'])
+            && strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) == 'xmlhttprequest');
+    }
 }