Issue #40: Add Views to Core (Cleaned up)

This PR ports the initial version of Views to Backdrop (https://github.com/backdrop/backdrop-issues/issues/40), temporarily using a custom key-value store until https://github.com/backdrop/backdrop-issues/issues/141 can be completed.

This PR replaces https://github.com/backdrop/backdrop/pull/107 with a cleaner pull request, with Views tags removed from the repository to trim down the overall repository size.