#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.

calling it 'default' was misleading since sometimes the legacy renderer
actually acts as the true 'default' (the renderer that's used when no
other one is specified).

callback property from 'render panel' to 'render region', changed some
variable names inside the plugins. Not all working yet, though.

renderer to determine what type of data it should be passing to style
plugins.

missing return that was causing nothing to be rendered at all.

default is at least as accurate, but is more common and doesn't have
some of the additional meanings 'standard' does.

any further; any TODOs apply to the standard renderer, or not at all.

last commit. Hmm...must have gotten too much beer from drewish.

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.

stack.

relevant docblocks.

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

updated appropriate docblocks.

respective docblocks.

panels_display object.

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.