merge r18967 from trunk to branch 2.4

bug:2687
bug:2778
Unable to switch from mobile theme to classic theme with incompatible plugins.
Detect blackberry and symbian for mobile theme.
Update mdetect class.

git-svn-id: http://piwigo.org/svn/branches/2.4@18968 68402e56-0260-453c-a942-63ccdbb3a9ee

include/functions.inc.php

@@ -1671,7 +1671,7 @@ function get_device()
   {
     include_once(PHPWG_ROOT_PATH.'include/mdetect.php');
     $uagent_obj = new uagent_info();
-    if ($uagent_obj->DetectTierIphone())
+    if ($uagent_obj->DetectSmartphone())
     {
       $device = 'mobile';
     }
@@ -1705,7 +1705,6 @@ function mobile_theme()
   {
     $is_mobile_theme = get_boolean($_GET['mobile']);
     pwg_set_session_var('mobile_theme', $is_mobile_theme);
-    unset($_GET['mobile']);
   }
   else
   {