Eliminate the need to click out of the flight level to allow continuing

[István Váradi]

When the user enters the flight level on the Route page, he/she has to leave the entry field so as to enable the Next button.

This should be improved, if possible, e.g. by performing the check after each digit entered into the field.

[István Váradi <ivaradi@…>]

In 122e92f35a2cc8284dab0e0d329f81c8fd496407:

The status of the Next button on the Route page is continuously updated while entering the FL on the keyboard (re #278)