Option added to delete airports

An option has been introduced which enables the user to delete
an airport. Upon deletion all routes associated with this
airport are deleted as well.
Option added to delete routes

The user can now delete rows by pressing the delete key. This removes
the currently selected row, and deletes the route associated with it
from the database.

Furthermore some changes have been made to the way airports are
deleted. Contratry to previous statements airports cannot be
deleted if one or more routes still exist which use this airport
as the start or destination.