fixed caching in a couple of places:

- user.php was not reloaded if (session-)preferences changed eg. language via select-box in login, because we used Expires header, but did not force a different url
- (user|config|images).php now has etag on url, to force reload by browser as we use an Expires header (changed images still need Admin >> clear cache to rebuild image cache)
- preferences are now loaded via a cachable GET request

json.php

@@ -100,7 +100,7 @@ if (isset($_GET['menuaction']))
 	{
 		throw new egw_exception_assertion_failed("JSON Data contains script tags. Aborting...");
 	}
-	$json->parseRequest($_GET['menuaction'], $_POST['json_data']);
+	$json->parseRequest($_GET['menuaction'], $_REQUEST['json_data']);
 	egw_json_response::get();
 	common::egw_exit();
 }