Backport of "gtkwaylandsink: Destroy GstWlWindow when parent GtkWindow is destroyed" into 1.22
Backport of !4197 (merged) "gtkwaylandsink: Destroy GstWlWindow when parent GtkWindow is destroyed".
Commits:
- 6ebd7867 gtkwaylandsink: Destroy GstWlWindow when parent GtkWindow is destroyed