Update GTK and GTK-layer-shell to latest versions

[Ferdi265]

This PR updates the GTK3 libraries to the latest versions, in preparation for the port to GTK4. This PR applies on top of #103.

The main changes are:

• use async_channel instead of the GTK MainContext::channel(), which has been removed
• slight changes in the GTK control flow enum (Break instead of Continue(false), I hope I understood this correctly)
• slight changes in the GTK-layer-shell API (trait methods on window instead of free functions)

[Ferdi265]

The async-channel fixes are incorrect; something breaks as soon as a notification from swayosd-client comes in, somehow closing the channel used for the libinput backend, which makes caps lock notifications fail. Will debug and fix this in the coming days.

[Ferdi265]

The async-channel fixes are incorrect; something breaks as soon as a notification from swayosd-client comes in, somehow closing the channel used for the libinput backend, which makes caps lock notifications fail. Will debug and fix this in the coming days.

fixed in latest push. Apparently we can't return from the closure we use the channel in, otherwise it gets closed somehow. Replacing return Continue(true) with continue works.