#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.

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.

phase responsibilities best practices.

parameter, as that's more inheritance/interface friendly.

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.

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

Whoops.

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

is now entirely moved into the prepare phase.

and therefore friendly to friendly to modification in child classes.

files.