Even though gtk4-layer-shell's documentation claims that "nobody quite
knows what it's for", some compositors (like Niri) can define custom
rules based on the layer name and it's beneficial in those cases to
define a distinct name just for our quick terminals.
As of now `gtk4-layer-shell` is unavailable on recent, stable releases
of many distros (Debian 12, Ubuntu 24.04, openSUSE Leap & Tumbleweed, etc.)
and outdated on many others (Nixpkgs 24.11/unstable, Fedora 41, etc.)
This is inconvenient for our users and severely limits where the quick
terminal can be used. As a result we then build gtk4-layer-shell ourselves
by default unless `--system` or `-fsys=gtk4-layer-shell` are specified.
This also allows me to add an idiomatic Zig API on top of the library
and avoiding adding even more raw C code in the GTK apprt.
Since we now build gtk4-layer-shell it should be theoretically available
on all Linux systems we target. As such, the `-Dgtk-layer-shell` build
option has been removed. This is somewhat of an experimental change as
I don't know if gtk4-layer-shell works perfectly across all distros, and
we can always add the option back if need be.
