[mobile ui] navigation rework

[Dirk Hohndel]

Please try again now. There was a small hickup in the backend
infrastructure that actually caused the problem. But in my testing the
latest apk works just fine now. Depending on how large your dive log is
(and how many pictures you have stored in the dive log - that seems to be
the biggest problem), it may take quite a while until the dive list is
initially loaded.

But after the first sync things move much more smoothly.

/D