* src/CMakeLists.txt: Define a macro when Gtk+ 3.22 is available.

* src/client/application/geary-application.vala (GearyApplication): Add
  show_uri method that uses the app's currently active window/screen to
  show the URI on, add a compile-time test to use Gtk.show_uri or
  Gtk.show_uri_on_window as appropriate, make the API nicer to use by
  not having two different error reporting methods. Rework existing uses
  of Gtk.show_uri to use this method.