The nested window is an xdg_toplevel client inside labwc, but tabbed-
wlroots never bound xdg-decoration-unstable-v1, so labwc added no title
bar. Generate the client protocol (wayland-scanner), register it in the
Makefile, and after creating the host toplevel request SERVER_SIDE mode
so labwc draws a real host title bar above the nested content. Adds an
informational configure listener that reports the effective mode.
