[proposal] Views on a property graph

Currently, there are many kinds of apps (immutable, arrow, flattened, projected, java-* )& fragments,
Maybe we need to reconsider do we really so many kinds of apps?
Introducing views will solve many issues:
- flattened/projected fragments and apps over them can be unified, projected_frag is a view by selecting a label, while flattened_frag is selecting all labels.
- apps always defined on a typed view, that's should be sufficient. (no need to define SSSP on a PG)