Make Layout Behaves Like Extension

[taufik-nurrohman]

Make .\lot\layout\index.php mandatory, and if it does not exist, the front-end view should output nothing. This allows Mecha to further becomes a “tiny web framework-ish” CMS rather than becomes a “has the potential to be full feature-ish” CMS that begins as a blogging CMS, just like WordPress.

It means that user can replace the built-in layout extension with their own layout system. Some may want to enable the old shield extension logic as in version 2.0.0, which allow users to store multiple layout files grouped in a different folder.