gstplayer: Correctly emit "seek-finished" signal

commit 2fa4b631 introduced a "seek-finished" signal by listening to the reset-time message. The problem with this approach lies in the fact that the seek operation is not finished. Thus, the position has not been updated yet and returns an undefined value.

Fix the issue by moving the signal emission in "_on_async_done" method. Indeed, an async-done message is sent once the seek operation is finished. A flag is set when requesting a seek operation to avoid sending a "seek-finished" signal when an other async operation has been performed.