Merge Panel, Panelizer Default & Panelizer Entity implementations

[algeronwaterschoot]

Panel import currently depends on an Export from the Panel UI window, and Panelizer has its own form for generating an export (though it makes use of DB queries that likely won't support modules which extend Panelizer).
The three implementations need to be modified so that they use a similar code-base, are all exportable from the same admin page and generate an import file that is sensitive to modules which extend Panelizer.