Convert loading of export/import to jQuery ready event.

This way it is done after document is fully loaded and no race
conditions can appear as with inline js.

patch #2984337, bug #2983446

libraries/plugin_interface.lib.php

@@ -370,6 +370,8 @@ function PMA_pluginGetJavascript(&$list) {
             }
         }
     }
+
+    $(document).ready(init_options);
     //]]>
     </script>
     ';