Add project property `release.tag` as a global `project.tag`

Add project property `release.tag` as a global `project.tag`

Add property `release.tag` to keep a generally available property.
The `project.tag` was getting used in many places, so I need a global
definition for it. The property `project.tag`, defined in the target
`tagRelease` could not be reused, since when that target runs, the
version is still a SNAPSHOT version. (And properties don't get
re-assigned.) So instead a new property `release.tag` is defined,
just like `release.name`, which can be only used sensible in a
release stage, i.e. after `tagRelease` ran.