Add support for watchPosition and clearWatch

[crankycoder]

The current implementation only supports getCurrentPosition.

watchPosition and clearWatch can be implemented with getCurrentPosition as long as we have an additional flag within the TrieLocator which monitors itself to see if it is in watch or one shot mode.

Note that this bug depends on having support for worker based scoping of the offline code.

It's possible that multiple pages are using the geolocation APIs and pages are using one-shot getCurrentPosition and watchPosition at the same time.