Issue #131: Wrong Arrow Position after Error Page

With current implementation, switching page from error 404 will only
load the page content without placing the page tab triangle arrow at
the correct position.

  In this patch, we introduce pub/sub mechanism in front-end to make
the structure loosely coupled. Each time an HTML fragment request is
being sent, a "redirect" event will be fired and the subscriber will
be notified to place the arrow at the correct position.

Signed-off-by: Hongliang Wang <hlwang@linux.vnet.ibm.com>