staging: add toplevel tag protocol

The window id protocol allows clients to set a tag for toplevels, which
the compositor can use to identify them even after the application
has been restarted. This persistent identification can be used by the
compositor to restore properties like position, size, "always on top",
and it can also be used for allowing users to create rules that change
compositor behavior for specific windows.

Signed-off-by: Xaver Hugl <xaver.hugl@kde.org>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>