[UX] Add the ability to make any standard layout template a flexible one

This is part of #4046, and a sibling-issue to #4115.

The idea here is to add a new "Clone" operation to all types of layout templates in /admin/structure/layouts/settings (both standard, as well as flexible). This would make a copy the configuration of flexible layout templates to a new one. For "standard" templates, it would parse their .info file, and create matching rows/regions.

This would effectively allow people to clone any "standard" layout template into a one-for-one copy of a flexible one. That way, they'd get a starting point, with regions already configured same as the "origin" template, and they'd from there be able to add/remove/reorder regions/rows as they please.