GTK2 demotion

[Matthias Klose]

Similar to the Python2 demotion analysis, there is one done for GTK2 by Jeremy
Bicha, at

  https://bugs.launchpad.net/ubuntu/+bugs?field.tag=gtk2-demotion

There are a few hard to fix issues, but please review the seeds if the input
helpers in the seeds can be updated to avoid the GTK2 usage.

This is not a goal for the upcoming 18.04 LTS release, however it would be
appreciated to address some of these issues for the LTS for easier maintenance.