gtk: implement quick terminal

Using `gtk4-layer-shell` still seems like the path of least resistance,
and to my delight it pretty much Just Works. Hurrah!

This implementation could do with some further polish (e.g. animations,
which can be implemented via libadwaita's animations API, and global
shortcuts), but as a MVP it works well enough.

It even supports tabs!

Fixes #4624.

nix/devShell.nix

@@ -31,6 +31,7 @@
   glib,
   glslang,
   gtk4,
+  gtk4-layer-shell,
   gobject-introspection,
   libadwaita,
   blueprint-compiler,
@@ -88,6 +89,7 @@
 
       libadwaita
       gtk4
+      gtk4-layer-shell
       glib
       gobject-introspection
       wayland
@@ -167,6 +169,7 @@ in
         blueprint-compiler
         libadwaita
         gtk4
+        gtk4-layer-shell
         glib
         gobject-introspection
         wayland