Move fragment navigation to activity
Closes #141 (closed)
WIP: based on !108 (merged)
WIP: want to fix #144 (closed) too with this MR on second thoughts, let's keep a solution for #144 (closed) out of this. We probably need to save the current lifecycle state to a bundle in onSaveInstanceState()
as I do in !110 (merged). Let's either work on #144 (closed) separately or try to add it to !110 (merged) where it might fit in nicely.
Edited by Sebastian