libweston: remove output_pending_signal

The signal has been replaced with the heads_changed hook and is no
longer useful.

weston_pending_output_coldplug() is renamed to
weston_compositor_flush_heads_changed() for two reasons: it better
describes what it does now, and it serves as an obvious flag that
libweston ABI has been broken.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>