libweston: Init weston_output's 'destroy_signal' before timeline has a chance to...

libweston: Init weston_output's 'destroy_signal' before timeline has a chance to emit a
timeline subscription

When subscribing over the command line to the 'timeline' scope we hit
the situation where we could emit a timeline message but without the
weston_output object being (fully) enabled.  The timeline subscription
object requires to install its own callback on the 'destroy_signal' but
at that time, the 'destroy_signal' is not initialized.

This moves 'destroy_signal' initialization before timeline has a chance
to emit a timeline subscription message for that weston_output.

While at it, move also 'frame_signal' initialization before any function
call to keep them nicely organized.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>