#841824 by levacjeep: Extend panels_edit and panels_edit_layout to allow more options with the $destination.

Improve the renderer selection framework. This framework has no UI, but it utilizes exportables and new choices can be hand crafted.

#844062: Turned the normal drag and drop editor into a renderer plugin, made the IPE part of it. Also some fairly serious code reorganization.

Deprecated panels_get_panels() function. This will be eventually disappear. Use panels_get_regions() instead.

restored with its original functionality.

Credit for this patch goes 100% to drewish for getting me beer.

This is a major change, though one that shouldn't affect any existing
Panels implementations (unless they're using region styles in a totally
unanticipated way). Rather than having style plugins with region support
call panels_render_pane() themselves on a pane content object, this
change moves that logic further up into the render pipeline and simply
passes the regions an array of already-rendered/styled/themed pane
output.

relevant docblocks.

logic with its old wrapper (panels_render_pane()) into a single method
on the renderer for readability.

updated appropriate docblocks.

respective docblocks.

encapsulated polymorphism (display renderer). Though the core effort
should probably employ top-level polymorphism, encapsulation is fine for
the current purposes, and makes backwards compat a LOT easier.

submit callbacks.

approach was old and crusty.

Introduce the Panels stylizer module which builds on the CTools stylizer base to create user definable, recolorable styles.

#612704 by c960657: panels_get_pane_content() inappropriately created a blank context when no contexts were available.

#624710: fix an old, old bug where the default empty context was not being appropriately wrapped in an array.

#607420: Visibility rules did not appear in the list when added to a pane (though they still applied).