Update gtk4, gstreamer, libshumate

This MR contains the minimum amount of changes to update gtk, gstreamer and libshumate to the latest version.

Even though libshumate is now compiled to use libsoup3, there's still the error

libsoup2 symbols detected. Using libsoup2 and libsoup3 in the same process is not supported.

I guess we will solve that another time... Everything else seems to work as usual.