add possibiliities of positionning the navbar (static, fixed, ...)

[JOduMonT]

Hi;

If I may; I would like to propose this option for this theme.

I tried my best to optimize the code, not disturb to much your logic and respect the bootstrap canvas. I still a early stage of learning twig and grav. So basically I just copy and paste from bootstrap4 template

this version also manage the submenu dropdown/dropup