[UX][D8] Merge the "List links" and "Edit menu" operations into a single administration page.

For Backdrop, we should build the functionality of simplified_menu_admin module into core. It combines the list of links with the title and description fields, so we can reduce the menu administration forms from 2 into one.

Here's how it would look (with the already ported simplified_menu_admin module):

Here's how the respective admin page looks like in vanilla D8 after https://www.drupal.org/node/663946 got merged: