New release - Githubissues

Previous release issue was #137.

Before next release we need:

[x] Update all crates to 1.56 and 2021 edition
[x] Change namespacing of NONE (https://github.com/gtk-rs/gir/issues/1252)
[x] #176
[x] Sync releases with gstreamer-rs (@slomo), relm4 (@aaronerhardt), libadwaita-rs (@bilelmoussaoui) and also mention those in the release blog post
[ ] gtk-rs-core: 0.15.0 milestone (https://github.com/gtk-rs/gtk-rs-core/milestone/2)
[ ] gtk4: 0.4 milestone (https://github.com/gtk-rs/gtk4-rs/milestone/4)
[ ] gtk3: 0.15 milestone (https://github.com/gtk-rs/gtk3-rs/milestone/2)
[ ] gstreamer-rs: 0.18.0 milestone (https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/milestones/2)
[ ] https://github.com/gtk-rs/gir/issues/1261

gtk-rs / release