Add a 'editor render callback' to pane content types

Stupid #267884: Attachments failing when users preview or use "attach" button ...

Whoops, syntax error in the last patch. This one is actually tested and working.

Here's a trivial example of what I need this for:

/**
 * Render a story pane for editing on the content tab.
 */
function sw_panels_story_edit_render($display, $pane) {
  $block = new stdClass();
  $node = node_load($pane->configuration['nid']);
  $block->title = check_plain($node->title);
  return $block;
}

Here's a more complicated example:

/**
 * Render an image pane for editing on the content tab.
 */
function sw_panels_image_edit_render($display, $pane) {
  $block = new stdClass();
  $conf = $pane->configuration;
  $node = node_load($conf['nid']);
  $block->title = check_plain($node->title);
  $size = !empty($conf['image_size']) ? $conf['image_size'] : '242';
  $block->content = theme('imagecache', $size, $node->field_image[0]['filepath']);
  return $block;
}

Ok, I'm re-rolling this to include a patch against docs/sample_plugin_ct.inc -- stay tuned.

I'm not 100% sure I got all that doxygen stuff right, but I think this is pretty close...

Note: please see #270559: After editing a pane, the "portlet" collapse/expand behavior is lost which is a bug made more obvious by this new feature... Any help on how to debug/fix that would be most appreciated. ;) Thanks.

New patch that re-invokes 'editor render callback' after editing a pane. This partly fixes #270559: After editing a pane, the "portlet" collapse/expand behavior is lost but not completely. I'm still investigating that one right now.