feat: add gtk and kde appmenu protocols

[81reap]

First step toward issue #832 (global application menus on Wayland). This PR adds two Wayland protocol XMLs into protocols/ and wires them into the meson build. My plan is to raise follow up PRs to build on top of this :

1. (this PR) Add the two protocol XMLs.
2. Implement org_kde_kwin_appmenu server-side: store per-Client state, register the global.
3. Implement the trimmed gtk_shell1 server-side. Same shape.
4. Bump dwl-ipc-unstable-v2 to interface version 3, add a single new appmenu event carrying both protocols' fields, expose them via mmsg -M, with backwards-compatible versions so old panels keep working.

Files in this PR

• protocols/kde-appmenu.xml (new) — verbatim copy from
  plasma-wayland-protocols, MIT licensed. Provenance + SPDX header inline.
• protocols/gtk-shell.xml (new) — trimmed subset of GTK's gtk-shell.xml,
  LGPL-2.1-or-later. Provenance + trim notes inline.
• protocols/meson.build — appends the two filenames to wayland_xmls.

Why the GTK XML is trimmed

Upstream gtk-shell.xml declares both gtk_shell1 and gtk_surface1 at version 7. The vast majority of that surface area is unrelated to menus — tiled-state CSD hints (v2), focus-stealing bypass (v3), titlebar gestures (v5), accessibility paths (v7), startup notification (v3), etc. Each would require a stub handler in the compositor.

To keep the implementation surface area minimal, this XML declares both interfaces at version 1, retaining only what's needed for menu publication:

• gtk_shell1: get_gtk_surface, capabilities event
• gtk_surface1: set_dbus_properties

plus the other v1 requests that must remain for opcode alignment with the upstream wire format (set_startup_id, system_bell, set_modal, unset_modal, present).

Verification

• meson setup build && meson compile -C build succeeds — wayland-scanner generates kde-appmenu-protocol.{c,h} and gtk-shell-protocol.{c,h} with no warnings.
• Both XMLs are well-formed (xmllint --noout).
• Smoke-tested that mango still launches and mmsg -g output is unchanged from main.