scenario: Mark seek as done only when reaching next state

scenario: Mark seek as done only when reaching next state

There is a race where following actions could generate a
flush-start/flush-stop dance but the state change resulting from the
seek hasn't been committed yet, leading to the ASYNC_START being
ignored by GstBin since its pending_state is not VOID when receiving
the ASYNC_START message.

Conceptually it is totally correct to consider an action done when
the state change of the pipeline is stabilized..

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-devtools/-/merge_requests/198>