Disable unused layout templates

Description of the need

I've just implemented hook_theme_registry_alter() in a custom block and checked the registry content to see long list of variables, including for unused layout templates. Disabling them on admin/structure/layouts/settings freed registry from unnecessary content and even-though human eyes might not notice, definitely made things slightly faster.

Proposed solution

I believe Backdrop must be shipped as performant as possible, so I suggest to disable all unused layout templates by default. Users can easily enable them if they need additional templates.

Additionally, we could add a pointer link on pages like admin/structure/layouts/manage/dashboard/configure as shown below: