Previously our code assumed that g_cancellable_cancel() the async DBus
calls to libfeedback would guarantee that the underlying operation would
not be performed (i.e. triggering or ending a feedback).
However the endless ringing exhibited in #470 shows this assumption not
to hold. Therefore we avoid using g_cancellable_cancel () completely and
default to waiting for the async operation to finish.
update_ring () now sets the target state by inspecting managed calls and
the main logic will now step towards the target state:
Changing from regular/loud to soft/quiet ringing (or vice versa)
requires we first end feedback before (re)triggering it.
Additionally the "is-quiet" and "is-ringing" properties are replaced by
a new "state" property to allow changing the combination atomically.
Closes: #470
