libweston: Make weston_seat release safe

libweston: Make weston_seat release safe

Ensure the server can safely handle client requests for wl_seat resource
that have become inert due to weston_seat object release and subsequent
destruction.

The clean-up involves, among other things, unsetting the destroyed
weston_seat object from the user data of wl_seat resources, and handling
this NULL user data case where required.

The list of sites extracting and using weston_seat object from wl_seat
resources which were audited for this patch are:

Legend:
N/A = Not Applicable (not implemented by weston)
FIXED = Fixed in the commit
OK = Already works correctly

== keyboard_shortcuts_inhibit_unstable_v1 ==
[N/A] zwp_keyboard_shortcuts_inhibit_manager_v1.inhibit_shortcuts
== tablet_input_unstable_v{1,2} ==
[N/A] zwp_tablet_manager_v{1,2}.get_tablet_seat
== text_input_unstable_v1 ==
[FIXED] zwp_text_input_v1.activate
[FIXED] zwp_text_input_v1.deactivate
== wl_data_device ==
[FIXED] wl_data_device_manager.get_data_device
[OK] wl_data_device.start_drag
[FIXED] wl_data_device.set_selection
[OK] wl_data_device.release
== wl_shell ==
[FIXED] wl_shell_surface.move
[FIXED] wl_shell_surface.resize
[FIXED] wl_shell_surface.set_popup
== xdg_shell and xdg_shell_unstable_v6 ==
[FIXED] xdg_toplevel.show_window_menu
[FIXED] xdg_toplevel.move
[FIXED] xdg_toplevel.resize
[FIXED] xdg_popup.grab
== xdg_shell_unstable_v5 ==
[FIXED] xdg_shell.get_xdg_popup
[FIXED] xdg_surface.show_window_menu
[FIXED] xdg_surface.move
[FIXED] xdg_surface.resize

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>