Support restricting the allowed orientations (UA wise)

- Move the UA default to a global place and use it from the app window.
- Give the UA default to the ScreenOrientationExtension ctor.
- Set the UA default as part of the JS source string.
- Enforce (according to spec) that any web facing orientation lock
  much be a subset of the UA defaults.
Merge pull request #1454 from kenchris/uadefaults

Support restricting the allowed orientations (UA wise)