[#33337] fix 'show_tags' in content category layout

'show_tags' is incorrectly taken from $displayData instead of $params
It is forced to always be true since it doesn't exist in $displayData ($displayData->get('show_tags', 1)).

Can be tested by setting Content Category param for show_tags to Hide.  They are always displayed.
Also cannot override the component-level setting in the menu item as the field does not exist there.

Patch corrects the layout file and adds the show_tags field to content category xml file.
Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33337