Improve addressable content: return pane OR content

Currently, ctools_get_addressable_content($address, $type) defaults to $type='content', but in Panelizer at least, we also have access to the $pane object from the renderer, and we might want to return that in addition to the rendered content.

(I'm specifically wanting to read the settings from a cache plugin to grab a TTL, but I could see a number of uses.)

A patch is attached to add $type='pane' to panelizer_panelizer_task_get_addressable().