Fixed bug with CM_WP_Base::add_shortcode()

Corrected CM_WP_Base::add_shortcode() to pass the Base class object to Shortcode::register() method as was failing type hinting.
Corrected type in PostType constructor.
Updated Shortcode to extend Element class & made factory methods static.
Corrected a couple of typos in Shorcode constructor
Corrected visibility of Shortcode::is_handled_by() method.
Updated Shortcode::is_handled_by() method to store handler callback.
Attempt to fix issue with display of InvalidCallbackException message.

Message was displaying the callback as 'Array' if it was an array.  This fix uses var_export to convert the array to a useful string, but seems that when the message is returned now, it displays up to the callback, & then repeats from the beginning of the callback message.  So the full message is displayed, but further downy the message than expected.