AudioPlayer: Use Weak ref counting in order to not keep the object alive.

We were passing a strong ref to the gstreamer callbacks which was causing
them to keep the `AudioPlayerWidget` struct alive even after the gtk widget
had been destroyed.
2 jobs for master in 23 minutes and 26 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #53912
flatpak

00:18:55

 
  Deploy
failed #53913
pages

00:04:30

 
Name Stage Failure
failed
pages Deploy
Package gstreamer-1.0 was not found in the pkg-config search path.
Perhaps you should add the directory containing `gstreamer-1.0.pc'
to the PKG_CONFIG_PATH environment variable
Package 'gstreamer-1.0', required by 'virtual:world', not found


warning: build failed, waiting for other jobs to finish...
error: build failed
ERROR: Job failed: exit code 1