Backstack full functionality

Backstack now works as expected:
- When we are down in subsections back-button takes us up the tree
- When we switch main category backstack is cleared so we dont go down another tree
- When we are on the top level of some tree back takes us to home screen view
- When we are on the home screen and the backstack is empty android takes us out of the app and back to home / previous application as expected.