Block library: feedbacks

Problem/Motivation

There are some hardcoded blocks which can't be placed, I think for display_builder_page_layout, this will be a blocker, src/Plugin/display_builder/Island/BlockLibraryPanel.php:

private const HIDE_BLOCK = [
  'help_block',
  'system_messages_block',
  'htmx_loader',
  'broken',
  'system_main_block',
  'page_title_block',
];

1: The help and system message should be available, either as blocks or as slot sources.
2: Currently the local tasks is implemented as a slot source (modules/display_builder_page_layout/src/Plugin/UiPatterns/Source/LocalTasksSource.php), the problem is that that way it does not obtain the config form from core/lib/Drupal/Core/Menu/Plugin/Block/LocalTasksBlock.php, so maybe need to make the local tasks available as block and no more slot source?
3: why local actions and local tasks, and page title slot source had been put into display_builder_page_layout? I understand that for main page content, but for the other they could theorically be used into entity view display or view, so why not move it into the main module?

Proposed resolution

Test to have blocks instead of slot sources.

Remaining tasks

User interface changes