Switch gtk4-client to use gtk-3.99.1
There was a compatibility breaking update between 3.98.3 and 3.98.4 already, but there's no further one when updating to 3.99.0. There's compatibility breaking update between 3.99.0 and 3.99.1 -> #43416 both requires and is needed for 3.99.1
Repurposing this ticket for the update to -> 3.99.1
Once we've finished gtk3x-client move to gtk-3.98 in #43052, next step is moving to gtk-3.99.0.
Likely there's going to be two kinds of patches related to this. Some of them will build with both gtk-3.98 and gtk-3.99.0 and can be applied as they get ready, while we still build against gtk-3.98. Others will be needed for building with gtk-3.99.0, but would break build with gtk-3.98. All the patches of this latter type need to go in simultaneously when we switch from gkt-3.98 to gtk-3.99.0. This meta ticket is about collecting list of such patches to push at once, and discussing about the whole of the switch to gtk-3.99.0 (instead of going through the same things in each and every individual ticket).
- #43071 (gtk3x: Link against gtk-3.99.0)