has_default_panel() does not check if a default display exists

Initial patch. The rest of the system needs refactoring accordingly as the API was extended with some new functions.

This patch causes problems when an entity is first Panelized, it makes it impossible to get to the page to modify the new display as it doesn't really exist yet.

There's a problem with the static caching that caused #2305681: Panelizer display used in Views preview, not on actual page output to occur.

Thanks for fixing my facepalm-worthy bug.

Reviewing the updated code, is there any reason to use the $entity_type in the keys for $this->displays_loaded, especially when get_default_panelizer_objects() is passed the bundle name? Need to test this.

    // If no check was performed already to see if displays exist for this
    // bundle, try loading them.
    if (empty($this->displays_loaded[$bundle])) {
      $this->displays_loaded[$bundle] = TRUE;
      $displays = $this->get_default_panelizer_objects($bundle);
      $this->displays = array_merge($this->displays, $displays);
    }

Fixes a problem where entities were not being displayed if the default was changed and the ':default' display didn't exist or was disabled.

Committed. Thanks for the help, Andrew!

A follow-up to resolve an issue during hook_entity_load().

Committed. Again.