I see actions for navigateToURL and navigateToSobject, etc, but not NavigateToLightningApp?
Lightning Apps are meant to be single page Apps, not multi-page apps. If a means were available to navigate from one app to another other than through the SF1 Menu, that would effectively make them multi-page apps. To the best of my knowledge, what you're asking for does not exist at this time.
navigateToComponent was the closest we had, and it's gone now. Right now your only option is to mount your Lightning Page or Component as a tab, and handle all navigation yourself by dynamically creating the components you desire.
Coming soon (hopefully) will be the ability to mount a lightning component as a
recordTab or as a quick action, but we don't have that yet.