This function is especially usefull when using the leaks tracer. It was removed
in commit e7a0543c.

This is a workaround to get the fix from PR #113 on branch 0.11 without
breaking the API.

This was accidentially removed while merging a change from master

The name is the identifier of the visualization that has to be passed to
gstreamer_player::Player::set_visualization().

Fixes #111

It was accidentially ignored before

This fixes various memory-safety issues caused by broken reference
counting. We have to handle pre-1.14 and post-1.14 differently in
constructors.

See https://bugzilla.gnome.org/show_bug.cgi?id=743062#c30

As such, make them FnMut and remove the Sync requirement from them. We
can only do this for the callbacks and not the signals, because the
signals can in theory be emitted from anybody (outside the object!)
at any time.

And add a comment to why we need it. No other struct with a to_string()
function is affected.

https://github.com/sdroege/gstreamer-rs/issues/101

Instead of StructureRef's to_string(), we were calling
ToString::to_string() in the Display impl. Which then called into itself
again.

Fixes https://github.com/sdroege/gstreamer-rs/issues/101