Add jQuery/mootools selection to session settings for keepalive functionality

See tracker item 	29963
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_id=8549&tracker_item_id=29963

Details
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Currently behaviour.php will autoload mootools framework if any session
management uses keepalive - such as adding the login module to a site or if a
user edits an article in the front end.

Some templates remove mootools before rendering, so when this keepalive script
is added - unless the template checks and removes it - the added script causes
an error because mootools isn't present, and Keepalive uses the window.addevent
mootools function.

Adding a session framework option in the admin that allows admin to choose
between mootools and jQuery means that this front-end error can be avoided for
templates that don't want to use mootools.

Because behaviour.php is a core library it cannot be overridden by the
template, so they either have to accept mootools being added, accept the error,
or write some code to strip out the added code before the page is rendered - 2
of these options mean that the keepalive behavior doesn't work.