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

the renderer plugin. This backdoor shouldn't see much use in the
initial IPE release, but the general pattern is one that our javascript
will follow moving forward. Therefore, including it now.

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

Clean up flexible. Attempt to reduce the use of unnecessary divs.
Add more ability to control flexible classes.
Add ability to fix flexible canvas to a set width.
Allow disabling of scaling for IE6 in flexible.

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

relying on it.

visibility' permission.

panels_display_save().