Cache country <-> ISO code mapping in both directions

Summary:
This is just a stop-gap measure to avoid reloading the entire country map
file for every call (which is a noticeable problem when using this in a
list view delegate on an Android phone).

The real solution would be converting the map into a directly usable
format that we don't need to parse, but before doing that I'd like to
understand how the i18n() call is supposed to work here...

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: dvratil, kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D14127