compositor-wayland: Refactor struct wayland_output::name usage

compositor-wayland: Refactor struct wayland_output::name usage

struct wayland_output::name was used but never initialized.
Also zxdg_toplevel_v6_set_title was only called for windowed outputs,
and some compositors let you see the client's name even when it is
fullscreen (GNOME Shell's Activities menu for example).

So rename struct wayland_output::name to struct wayland_output::title and
precompute it on wayland_output_create_common(), so it can be later used
on xdg's set_title and frame_create.

v2: Move zxdg_toplevel_v6_set_title() before the wl_surface_commit()
as per Quentin Glidic's suggestion.

Signed-off-by: Sergi Granell <xerpi.g.12@gmail.com>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>