Sort the list of countries in the combobox.
Summary:
It's really confusing for the user to see a list of countries
sorted by (hidden) ISO code, i.e. not sorted by name, when looking
through the list for a specific entry.
Test Plan: itinerary -> Settings -> open Country combobox
Reviewers: vkrause
Reviewed By: vkrause
Subscribers: kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D14771