Rather than using 'begin_cb' rename it to a more suitable name.

Further more instead of using the scope use the subscription to pass as
an argument. The source scope is attached to the subscription when
creating it so we can access it that way.

This also adds a _complete and a _printf method for the subscription
such that the callbacks can use to write data to only _that_
subscription and to close/complete it, otherwise writing to a scope
results in writing to all subscriptions for that scope which is not
correct.

In the same time, the scope counter-parts of _write and _complete will
now use the subscription function as well.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Suggested-by: Daniel Stone <daniels@collabora.com>