Allow contrib to add support for any entity type

Here is a first stab at it. This patch adds a new function:

/**
 * Provides the content element key for a display context.
 *
 * This allows entity modules to specify their content element for field group
 * support, or other modules to add entity module support.
 *
 * @param $context
 *   The display context (entity type, form or view).
 */
function field_group_get_content_element_key($context = 'default')

And uses that to get the default content variable keys - 'content' or 'user_profile' - in field_group_build_entity_groups() and field_group_fields_nest(). The latter adds a third optional parameter, $context, to allow passing in the display context. Wherever $context is mentioned in existing code comments, those are updated to indicate $context may be the entity type, and it is this that is used to provide a corresponding content element key in field_group_get_content_element_key().

Finally, hook_field_group_content_element_keys_alter() provides a way for other modules to add entity-type to content-element-key mappings. This means that entity modules can easily add fieldgroup support themselves, or crucially, any other module or theme may add fieldgroup support on behalf of any entity module.

For example, to add fieldgroup support for commerce product variation entities:

function MYMODULE_field_group_content_element_keys_alter(&$keys) {
  if (!isset($keys['commerce_product_variation'])) {
    $keys['commerce_product_variation'] = 'product_variation';
  }
}

We could modify this to allow modules to specify the $element variable as well.

I also think the $variables['user_profile'] key is no longer correct for user module in Drupal 8. That would be a followup issue.

@karlos007: message the maintainer directly and ask him to reopen this and assign to 8.x-3.x. It's unlikely @zuuperman pays attention to closed/fixed issues.

@karlos007: This patch is already in 8.x-3.x. You need to implement the appropriate hook to specify the correct Commerce element keys, as per the example in #2.

Ideally, Commerce itself would implement this hook. The discussion is #2906502: Compatibility with Field group module