* ui/conversation-viewer.ui,
  src/client/conversation-viewer/conversation-viewer.vala: next-match and
  previous-match signals are only available in GTK+ >= 3.16.

* ui/empty-placeholder.ui: Drop declared GTK+ requirement to 3.14.

* ui/geary.css: The :not() pseudeo class is only available in GTK+ >= 3.20.