AppProxy with permissions checks and app context

Ref #2059

- Refactor appProxy into class that is instantiated per App
- Check for permissions before doing proxied filter/helper calls
- Add all currently existing api methods, let api check for permissions
- Basic unit tests for filter and helper register/deregister
- Adjusted proxy api method existence unit tests
Merge pull request #2631 from jgable/appProxyContext

AppProxy with permissions checks and app context