[WIP]: Extract main menu generation into NavbarGenerator.

This is a WIP for the UI plugins. Goal of this branch is to make it possible to dynamically (w/o rewriting parts of ManageIQ) add menu sections and menu items.

* Introduce a class for menu generation. Menu sections, menu items.
* Simplify navbar view.
* Define navbar in a declarative way.
* Remove redundant constants.
Struct.new can take a block, so you don't have to reopen the class.