[UX] Add visibility conditions for menu location

Description of the need I need to place a block in the sidebar on backdropcms.org that will only appear when the page is in a specific section of the menu.

For example: I have a "Developer documentation" block that I only want to appear in the "Developer notes" section of the "Handbook" menu. Or, where the "Developer notes" menu item is an ancestor of the current page.

Proposed solution I would love to have a core visibility condition for menu section that would allow me first to select a menu, and second, select an item that would be the ancestor of the page I would like the block to appear on. Let's maybe not use the word "Ancestor", the word "Section" should suffice here...

In Drupal 7 this could be accomplished by using the context module, or by using a custom query like this: ``` if (arg(0) == 'node' && is_numeric(arg(1))) { $result = db_query( "SELECT mlid FROM inet_menu_links WHERE link_path ='node/".arg(1)."' AND menu_name = 'name-of-the-menu'"); if ( $result->rowCount() >0) {return TRUE;} else {return FALSE;} } else { return FALSE; } ``