Bug#920589: CoqIDE supports GTK 3

[John Scott]

Control: unblock -1 885677
Control: forwarded -1 https://github.com/coq/coq/pull/9279
Control: tags -1 fixed-upstream

Unfortunately CoqIDE wasn't rebuilt for Buster. CoqIDE 8.10 supports lablgtk3 
though so it can be reintroduced