Add location validation support so that Page can provide mulitple target alternatives

[roryprimrose]

There are two problems with the current location verification.

1. Does not support multiple possible target locations without overriding IsOn
2. Does not support custom data in locations because locations are defined against the page type rather than the page instance

Solutions to this are to provide the framework for validating multiple target locations so that a derived page only has to define the possibilities, not the validation logic.

The second point is already taken care of in terms of navigation because the specific location can be requested via the Browser class. The solution above for #1 will then also solve the validation of custom data in locations.