libweston: Enable logging for libseat launcher

The built-in backend of libseat requires users to enable a logging
level in order for libseat to start writing out log messages.  For that
to happen we split out the info and error log level messages into the
compositor's log scope, while debug level messages go into a dedicated
scope.

With that, this patch brings in a new scope, called libseat-debug, which
users need to explicity create a subscription for it as to retrieve/have
access to debug message coming out of libseat. Note that by default we
have a subscription for the log-scope so any errors/info from libseat
would be displayed to the user.

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