Add flight booker example from 7gui

[sudormrfbin]

This is an example that emulates an application that books flights, as described in task 3 of 7gui tasks.

The focus of Flight Booker lies on modelling constraints between widgets on the one hand and modelling constraints within a widget on the other hand. Such constraints are very common in everyday interactions with GUI applications. A good solution for Flight Booker will make the constraints clear, succinct and explicit in the source code and not hidden behind a lot of scaffolding.

[sudormrfbin]

Initial state	Invalid date format	Return date before start date

[dzhou121]

Thanks!