Organize our work wrt. GTK+ 3.24.x release schedule

Apart of the GNOME Shell MR, all our pending MRs are against GTK+ 3.x (#15226 (closed), #15724 (closed), #15663 (closed), #15667). We (or at least I) don’t know what’s the timeline for a new GTK+ upstream release yet so I have no idea:

what’s our deadline for getting these merged
whether GTK+ 3.24 has a chance to make it into Buster

I think the way we organize/prioritize our work greatly depends on the answers to these questions.

IIRC we’ve briefly discussed this topic over email with segfault a couple weeks ago but I can’t remember if we reached an actionable conclusion.

Parent Task: #14468

Original created by @intrigeri on 15761 (Redmine)

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