Depth widget minor UX improvement

Decrease depth by tapping left part of widget. Icon is showing "up" (i.e. closer to surface).
Increase depth by tapping right part of widget. Icon is showing "down" (i.e. deeper).

Fixes #354

Signed-off-by: Andrey Zhdanov <andrjufka@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>