Reduce HTML duplication by introducing a layout

On each page there will be somethings that are the same. This would be the page header, main navigation, and stuff like that. So instead of duplicating that on each page we have a special "layout" view. This view will then contain all the stuff thats the same on every page and the include all the stuff thats different.

You just use `new View("some/path/to/view.php")->render()` as usual and edit `views/layout.php` to change the stuff thats shared between all pages.