[Sidenav]: component api restructuring

[mart-sessman]

Problem

currently there are two issues:

• html semantics are not correct. aria role and display style are mitigating this, but still need semantically correct solution
• sidenav design is too rigid, there is no possibility to add own components (header, footer e.g.)

Solution

• change structure to be semantically 100% correct
• menu items should be inside a container component not direct children of navbar itself