changing way to create AppFeatureViewModel

per the new API in DomainObjectContainer.
(The existing code ought also to work; have seen MissingMethodException, though, so something is awry somewhere...)
depending on Isis 1.7.0; update to README in preparation for release of 1.7.0 of this module
bumping to 1.7.0
bumping to 1.7.1-SNAPSHOT
closes #1
closes #2
closes #3
#4 - updating to 1.8.0-SNAPSHOT.