Persist decorations on workspace send

[JLErvin]

This change addresses https://github.com/JLErvin/berry/issues/143, (re)adding support for persisting decorations when windows are sent between workspaces. Although this was included previously, it was removed (unintentionally) as a consequence of https://github.com/JLErvin/berry/pull/136.

This change is implemented by moving the client_decorations_destroy call from client_delete to handle_unmap_notify. #136 was implemented to avoid BadDrawable errors when a window is closed, so we can move the logic there and allow the client_delete code to persist the decoration state.