xwm: dump_property() to use FILE internally

Write the output of dump_property() out in one log call. When multiple
processes (weston and Xwayland) are writing to the same file, this will
keep the property dump uninterrupted by Xwayland debug prints.

This is also preparation for more development in the same direction.

Signed-off-by: Pekka Paalanen <pq@iki.fi>
Signed-off-by: Maniraj Devadoss <Maniraj.Devadoss@in.bosch.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Daniel Stone <daniels@collabora.com>