Changing $this->input->get('layout') from the default "CMD" filter to "STRING"

Changing $this->input->get('layout') from the default "CMD" filter to "STRING" since the layout is allowed to contain a ":". This is in case of an additional layout in a template where the layout is stored as "templatename:customlayout".

This is currently an issue if you want to create an alternate layout for the categories view in com_content.
The bug doesn't appear in all views because there are checks for the current active menu item layout built-in. 

However it never works if you use the URL parameter "&layout=templatename:customlayout".

Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=31494