v2.16

CHANGES:

• Update to GTK 3, because Debian is removing GTK 2 support now.
  Note that the systray icon no longer blinks to indicate that action is
  required, as GTK removed this feature. This also updates the static Docker
  images to use Ubuntu 16.04, since 14.04's version of GTK 3 is too old.

• Upgrade to dune 2.1.