Fix multi-line strings in QML files
Due to unescaped literal newlines, the strings were not being properly extracted into .pot files by xgettext (only the first line in each string was extracted). With proper escaping, the newlines disappear, so each \n must be doubled (so that each ‘line’ becomes a separate paragraph).
See also the discussion here: https://mail.kde.org/pipermail/kde-i18n-doc/2023-January/001379.html and continuing here: https://mail.kde.org/pipermail/kde-i18n-doc/2023-July/001654.html