Obsolete IStatePropertyAccessor

[tracyboehrer]

Fixes #26
Fixes #27
Fixes #28

This PR also reorganizes and adds some additional unit tests. The meaty part of this PR is in the Microsoft.Agents.State and Microsoft.Agents.BotBuilder.Dialogs packages. The high number of file changes is mostly in tests.

Of note:

• This PR annotates the BotState.CreateProperty with Obsolete. This allows bots migrating from BF SDK to work without change in this regard. Then we remove IStatePropertyAccessor functionality next major version. i.e., This is a friendlier migration path, though making the change is not difficult.
• There will be some additional ideas around this presented in subsequent PR's for discussion.
  • Auto Save (without using AutoSaveMiddleware). The current pattern is bot devs override OnTurnAsync and call save. This could happen more automatically.
  • Dotted property access. ex. BotState.GetProperty("mystate.foo.bar"), or BotState.GetProperty("mystate.list[myState.activeItem]"). This is something the Teams AI folks use since it plays nice with prompt templating engines.