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.

  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.

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.

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.

assorted bits.

toggling.

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

right place.

class.

Also added the renderer & menu callback for a single-pane render.

Other minor touchups.

Gonna take a lot more elbow grease before it all lines up, though.

actually functional)

calling a theme wrapper on processed pane output to allow arbitrary
wrapping around it.