player: Fix signal emission after a seek operation

As the seek_simple operation is asynchronous, the seek-finished signal
cannot directly be emitted by the Player.

Fix the issue by listening to the reset-time message in
GstPlayer. This message is sent once the seek_simple operation is
finished. The seek-finished signal is then transmitted to the Player.
3 jobs for master in 7 minutes and 47 seconds (queued for 1 second)
Status Job ID Name Coverage
  Check
passed #320685
flake8

00:01:41

 
  Build Flatpak
passed #320960
flatpak

00:06:05

failed #320686
flatpak

01:49:25