The API for expanding the panels module comes in two pieces. First there is the 
layout API, which adds to the list of layouts you need. Second is the content
types API, which lets modules supply content to the panels. Natively, panels
module supports the content types of 'block', which just renders the output
of a block, 'node' which simply renders a node_view, 'custom' which allows the
user to enter custom content with filtering, and finally 'views' because I
wrote them both.

Where to put your code:
=======================

With both types, there are two ways to implement a new type. First, you can
implement the hook in your module and provide the necessary data. Or you
can create a .inc file in the right format, and drop it into the proper
directory in the panels module. Both are very similar, and only requires
a minor naming adjustment.

When using the .inc file, in place of 'hook' in the various hooks, use
panels_FILENAME. 

Creating a new Layout Type:
===========================

A layout consists of 4 things:

1) A bit of HTML in a theme function. I use heredoc notation to make it
   extra easy to convert these to .tpl.inc files in case they are to
   be overridden in php template.
2) A bit of CSS to describe how the layout should be, well, laid out.
3) An icon that is 50x75 which gives the user a visual indication of
   what the layout looks like.
4) An implementation of hook_panels_layouts() to tell panels the necessary 
   information.

hook_panels_layouts returns an array with the following information:

'module' => The module name providing this. This is necessary because it
            uses drupal_get_path('module', $module) to get the proper
            path for included CSS.
'title'  => The title of the layout presented to the user. Use t().
'icon'   => The filename of the icon to use when listing avialable layouts.
'theme'  => The theme function that contains the HTML, without the theme_ 
            part.
'css'    => The CSS file.
'content areas' => an array in the form of 'name' => t('Title') of content
                   areas supported by the layout. For example, the simple
                   2 column layout uses array('left' => t('Left side'), 
                   'right' => t('Right side')); -- the name is the internal
                   identifier. Your theme function will see it as 
                   $content['name'] (so twocol gets $content['left'] and
                   $content['right']).


Creating a new Content Type:
============================

Content types require 1 hook and two callbacks. The hook defines what content 
types are available, the first callback displays the content in a dashboard, 
and the other callback does all of the administrative functions.

hook_panels_content_types returns an array with the following information:
'callback'  => The function to display the content.
'admin'     => The function to administer the content.

The callback function receives one argument: The $configuration array, as
defined by the administrative callback.

The administrative callback recieves 3 arguments:

$op     -- the operation to perform. See below.
&$arg1  -- The first argument should be a reference and its meaning varies 
           based on the op.
$arg2   -- The second argument is optional, not a reference, and should 
           default to NULL.

Administrative operations:

'list':     $arg1 is the configuration array.
    This op is called when panels lists what content is in a content
    area. It generally returns something similar to this:
    return '<strong>Views</strong>: ' . $view->name . ' (' . $view->description . ')';

'add button': arguments not used here.
    This op is called to display the 'add content type' button; it can also
    display additional information (such as the list of blocks or the
    autocomplete to select a node).

    The actual button should look something like this:
        $form['submit'] = array(
          '#type' => 'button',
          '#value' => t('Add view'),
        );

    And it's a good idea to do this, but it's not required:

        $form['#prefix'] = '<div class="container-inline">';
        $form['#suffix'] = '</div>';

'add': $arg1 == the $configuration array
    This op is called to see if your add button has been clicked. It *must*
    start off by checking to see if this is true:

        if ($_POST['op'] != t('Add view')) {
          return;
        }

    If it is true, it should process that information and return a $configuration
    array populated from whatever other form items were presented in 'add button'
    and whatever defaults make sense.

'edit': $arg1 == the $configuration array
    This op is called to provide an edit form for a content type. It *must*
    ensure *all* information from the conf array is available, even if it
    is just hidden; panels has no way to remember this data between form
    clicks, so any data not put here will be lost. No buttons need to be
    added to the form.

'validate': $arg1 == $form_values, $arg2 == $form
    Called to validate the 'edit' form above.

'save': $arg1 == $form_values
    Called to convert a $form_values back into a $configuration array. All
    of the default types just send $form_values back as $configuration,
    but if you need to do some kind of transformation, this is where it
    happens.