This has most notably impact in selection buffers, because those were
shared across all selection atoms. This turned out wrong on 2 situations:
- Because the selection atom was set at SelectionBuffer creation time, the
  GDK_SELECTION_NOTIFY events generated will have unexpected info if the
  buffer is attempted to be reused for another selection.
- Anytime different selections imply different stored content for the same
  target.

This is better separated into per-selection buffers, so it's not possible
to get collisions if a same target is used across different selections.

https://bugzilla.gnome.org/show_bug.cgi?id=768177