Menu support

[thmarx]

We use the Filesystem as representation of the navigation. What if we use some kind of file , JSON or yaml to build menus.
So we could have different menus, reuse content in different menu items.
Different menu items types would be possible. LinkItem, ListItem or External LinkItem

[thmarx]

This would have a big influence to nearly all parts of the application.
Content resolving has to use the menu data to
My first opinion, it speaks against the simplicity we want.

[thmarx]

the menu doesn't change the structure or folders, its just the menu at the page.

we need some kind of pagebuilder to create all kinds of menus.