Support Panels-style layouts

Context's support for multiple layouts, each with a distinct set of regions, is key to its flexibility and power in providing diverse page designs. But since this functionality depends on layouts being provided by the current theme, it faces two key limitations: 1., relatively few themes (yet) support layouts, 2. even if layouts are more broadly introduced, each theme may provide a distinct set of layouts with different regions, meaning that features built on context will still have difficulty relying on layouts.

Enabling modules to provide layouts could address these limitations. But what would a module-supplied layout consist of? Our best existing model would of course be Panels. But unlike Panels, with Context we need to support content added to the main content area as well as content written to layout regions.

Possible approach:

• Modules declare their layouts using the same array format currently used by themes, but in hook_context_layouts().
• Patch context_layouts_reaction_block.inc to recognize and handle module-supplied layouts alongside theme-supplied ones.
• If they don't use a template, modules are responsible for rendering of their own layouts, if the layout is set as active.
• A module-supplied layout is assumed to include the regular $content variable content as passed to theme_page() and to render all its content in the place of the regular $content output.
• Context layout reaction configuration includes the option to suppress regular theme regions aside from those in the current layout (as is currently done with theme-supplied layouts).

Sample hook implementation:

/**
 * Implementation of hook_context_layouts().
 */
function example_context_layouts() {
  $layouts = array();
  $layouts['example_2col'] = array(
    'name' => t('Example two columns'),
    'description' => t('A two column layout'),
    'stylesheet' => 'example-2col.css',
    'template' => 'example-2col',
    'regions' => array(
      'example_left',
      'example_right',
    ),
  );
  return $layouts;

}

Or I guess if the module wasn't using a template:

/**
 * Preprocessor for theme('page').
 */
function example_preprocess_page(&$vars) {
  if ($layout = context_layouts_get_active_layout(FALSE)) {
    $vars['content'] = theme('example_layout_' . $layout, $vars['content']);
  }
}

A module-supplied layout isn't as powerful as a theme one, as in most themes it could control only a portion of the page and could produce theme layout issues. But it would make it possible to have a consistent set of regions throughout a feature set.

Feasible? Desirable?