First steps towards a decent menu

# Progress

* [x] Place menu items on top
* [x] Add decent menu background
* [x] Make sure the header is above the menu background
* [x] Add burger menu icon
* [ ] Add as much direct menu items as fit in the menu bar
* [ ] Create slide open menu on the right side, interacting with the burger icon
* [ ] Add decent side menu content
[PSR-2](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md) error detected. Each PHP statement must be on a line by itself. Please terminate this defect.