Fix #3: Administration unavailable under XH 1.7

Since CMSimple_XH 1.7 doesn't import arbitrary global variables from
the query string anymore, we have to use XH_wantsPluginAdministration()
to detect whether the plugin administration has been requested. For BC
we also keep the old check for the global variable as fallback.

admin.php

@@ -25,7 +25,8 @@
 }
 
 initvar('calendar');
-if($calendar){
+if(function_exists('XH_wantsPluginAdministration') && XH_wantsPluginAdministration('calendar')
+   || $calendar){
     $plugin = basename(dirname(__FILE__),"/");
 
     //Helper-functions