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

  Add pane would add the new pane outside region styling
  Add pane would not appear in empty regions
  Panes could not be dragged to empty regions
  Save would lose content if they were inside stylizer styled regions

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

state. Very hacky for now, needs much improving.

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).

within the body of the $ alias checker as it's silly.

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

by the main Panels module, as nothing about it is actually specific to
IPE.

system name of a panel region, and $region_title refers to the
human-readable name.

now-extendable design; initial work towards a better system for placing
the 'add new pane' button, and placeholders for empty regions.

that cause fatal errors to fail more safely.

and therefore friendly to friendly to modification in child classes.

jQuery target

files.

what's going on well enough to do a proper fix.

will have to be significantly revisited later, but binding to page
manager permissions is safe, and therefore sufficient, for now.

content from the wrapper and the theme/plugin-generated markup

self-explanatory

assorted bits.

toggling.

through multiple editing start/stops, as needed.

level (i.e., wrapping the output of the entire panel display)

footer - simply zero likelihood that the IPE gets extended in that kind
of way