This fixes a bug when using the <kbd>Home</kbd> and <kbd>End</kbd> keyboard shortcuts to navigate through a `Menu` instance. These two buttons didn't update the `#lastIndex` field, which means that e.g. a following <kbd>ArrowDown</kbd> or <kbd>ArrowUp</kbd> press could make focus "jump" to an unexpected menu-item.
Also, the helper method reduces a little bit of code duplication in the event handlers.