Add libadwaita as a Meson wrap
Wraps are easier to work with than Git submodules and Meson has built-in support for them.
Tried to add GTK4, GLib, and xdg-desktop-portal as wraps, but GLib and xdg-desktop-portal won't work because the minimum Meson version is too low. GTK4 I think needs meson.override_dependency()
because of libadwaita, and that requires Meson >= 0.54. This will get the job done for now though.