Replace `unstable` with `staging`

Time has told us that the effort going from `unstable` to `stable` is
enough of a burdon meaning very few protocols are ever declared stable.

To mitigate this, and thus avoid having protocols being "stuck" being
"unstable" indefinitely, replace the "unstable" -> "stable" procedure
with a "staging" -> "stable" procedure, where declaring a protocol
stable does not involve any changes to any implementations.

The only side effect of this is that version numbers are to forever be
part of all interface names and protocol XML files.

Closes: wayland/wayland-protocols#30



Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>