Changes in VerticalSelector to simplify the way models are set

List of objects used as models were removed, using int instead.
Cleaning the API to not expose the list index. Updates incidence-editor to follow these changes.

This commit solves an issue with selector showing a number and the calendar UI showing a different month.

Simplifying this code and improving other parts in future commits, we expect the crash reported in 253299 won't happen anymore, and also fix weird things of the date selection UI.