Navbar improvements

Here is a list of simple improvements to Navbar component.
Please see the commit messages for more information about each proposed change.
I'm not very happy with this new method, because there is also `Navbar#componentId()`.
The application can also use `navbar.addComponents(new NavbarText("myRandomId", "Plain text").position(Navbar.ComponentPosition.RIGHT));` too. It just needs to make sure it is unique for the Navbar.