[ticket/13048] Only do not update the session page for ajax requests

The session time still needs to be updated, so e.g. long upload times
with PLupload stop the session from timing out.

PHPBB3-13048
Merge pull request #2985 from nickvergessen/ticket/13048

[ticket/13048] Only do not update the session page for ajax requests

* nickvergessen/ticket/13048:
  [ticket/13048] Only do not update the session page for ajax requests