Widget Admin: Auto-generated IDs Can Duplicate Page IDs

[ssmathias]

When a widget area is registered with a name that matches the ID of one of the standard admin page elements, it creates a duplicate ID which gets targeted by JavaScript, resulting in unexpected behavior.

For example, registering a widget area with the name of "Footer" will create a widget admin panel with the id of "footer", matching the existing id "footer" used for the admin page's footer content.

I propose namespacing the auto-generated elements with "widgetarea-" to avoid creating duplicate IDs with admin core elements.

[nacin]

#14466