Add Page.getLocation() (#9249)

Add Page.getLocation() (#9249)

* Send the whole window location, not just fragment, in bootstrap and when the fragment changes
* BrowserDetails now has URI getLocation() instead of String getUriFragment()
* Keep FragmentChangeListeners as-is, should perhaps change to LocationChangeListeners at some point
* Implement Page.getFragment() by means of Page.getLocation()
* Differentiate between no fragment (null) and empty fragment ("") as java.net.URI does

Change-Id: I1da1ea0664304d0c121a57e85d127fe48605e940