Language selection screen refactoring

This commit drops tracking of `usr/share/live-installer/{countries,languages,locales}` and instead relies on standard, system-provided, up-to-date alternatives, namely packages `isoquery`, `locales`, and `iso-codes` (15MB, but likely installed as some dependency anyway). Also replaces `usr/share/live-installer/flags/16/*.png` with 16x11 PNG flags from `famfamfam-flags-png` package. Finally, TreeView headers have been made sortable to convenience (entertain, and educate!) the odd user. :globe_with_meridians: