Add support for dropdown menu instead of treeview

[EitanBlumin]

Use jQuery implementation (see #117 ), and something like the following:

https://stackoverflow.com/questions/51871908/bootstrap-4-nested-dropdown

[EitanBlumin]

Should probably prepare more generic wrapper functions, or several different functions, each receiving a JSON / JS object as parameter representing the navigation tree, and using jQuery to generate a navigation bar, based on layout settings (sidebar, topbar, Bootstrap 4, Bootstrap 3, etc.).

And/or maybe use some kind of string templating methodology to make these functions as flexible as possible.