Fix JMenuBarFactory

This replaces the dirty HashMap implementation of accessing the "Forwarding" menu item by simply using a reference passed to the respective `Command` instance

It also combines `JMenuBarFactory` and its only descendant class `MainFrameMenuBarFactory`.

Since the calls to the HashMap are removed, I also added a secondary version of `addMenuItem()` which includes adding the item's command to the `EventCommandLookUp` map, which is the default use-case.

This fixes issue #13.