libweston: Send seat name before announcing devices

Clients need to know the seat name at the time they create mouse and
keyboard objects. This brings Weston in line with other compositors.

The documentation upstream currently is not super clear. It states name
is explicitly sent on bind, capabilities don't mention being sent on
bind in any way.

Signed-off-by: David Edmundson <davidedmundson@kde.org>