Making the app run again by giving it a persistor
Implement ActivityInventory#change_activity

* one of the many possible ways to always save changes to an activity
* all calls in the GUI that change an activity will now have to be wrapped
  in a change activity block
* I thought that this is better then building my own little AOP solution and
  I wasn't content with the existing ones
further specs for the interaction of the inventory witht he persistor duck type
init_slot just has to do nothing
Removed unused return and useless comment
Incorporated change_activity into the GUI
We dont need no Home Tav, we rather need todo today
List of finished activities and some clean_up
Forgot some wrapping...
Refactoring to move the button logic inside the ActivityTableSlot