[planned] Country selection hierarchy

[gyboth]

We have so many different countries now that the list is really overwhelming. We should sort them into some kind of continental hierarchy, e.g. Europe, North America, South America, Asia, Oceania and Rest.

The sorting should be reflected in the support_files/definitions/ folder structure on the one hand, and in the starting window UI on the other; the latter should use something like a dynamically generated (based on the folder structure, which has to be scanned beforehand) GtkComboBox or GtkMenuBar.