Show a LOCK icon on the page list when pages are locked for editing either by the current user or another user.

#589342: Introduce page_manager_get_current_page() to get information about the current page manager page.

- php potx-cli.php --mode=multiple
- msgcat views_content-plugins-content_types.pot views_content-plugins-views.pot views_content.pot --use-first --no-wrap -o views_content.combined.pot
- msgcat page_manager-plugins-tasks.pot page_manager-theme.pot page_manager.pot --use-first --no-wrap -o page_manager.combined.pot
- msgcat ctools.pot includes.pot plugins-access.pot plugins-arguments.pot plugins-content_types-block.pot plugins-content_types-custom.pot plugins-content_types-form.pot plugins-content_types-node.pot plugins-content_types-node_context.pot plugins-content_types-node_form.pot plugins-content_types-term_context.pot plugins-content_types-user_context.pot plugins-content_types-vocabulary_context.pot plugins-contexts.pot plugins-relationships.pot --use-first --no-wrap -o ctools.combined.pot

Make sure that tasks that will not override existing overrides refuse to enable and provide an error message saying why.