#651 Review / implement cleanup of all resources

[Christa Runge]

Since using the UI state restoration API, it is no longer possible to use the view controller hierarchy for a complete cleanup of all resources, because the view controllers must stay alife longer now.
On the other hand, we are using ARC memory management, which automates most of the cleanup of view objects, if memory properties are setup correctly.

However, a few resources still remain that must be explicitely cleaned up at application termination: the WkWeb view; our database; all memory that we alloc with plain C syntax; and possibly some of the notifications (timers / keyboards / ...)

The purpose of this tricket is meant as reminder to not forget to implement and review this aspect.