Stop support for GTK2
Quick summary of mail discussion:
This issue is to stop supporting GTK2, the constraint is to stay compatible with Debian stable. The original issue was triggered by commit 9a47cae5 which is incompatible with lablgtk <= 2.18.6. For the record, Debian is using version 2.18.5.
In this issue, we should check if the Debian stable package for GTK3 supports Why3:
- if it does, disable GTK2 entirely,
- if it does not, revert 9a47cae5 and other potentially failing commit.