dialogs: use semantic markup instead of dlgsec stuff

classes/article.php

classes/backend.php

classes/dlg.php

classes/feeds.php

classes/pref/feeds.php

classes/pref/filters.php

classes/pref/labels.php

classes/pref/prefs.php

classes/pref/users.php

css/default.css

css/default.css.map

css/tt-rss.less

plugins/af_psql_trgm/init.php

plugins/af_readability/init.php

plugins/import_export/init.php

plugins/mail/init.php

plugins/mailto/init.php

plugins/note/init.php

plugins/share/init.php

themes/night.css