This event is emitted after a layer's location property
has changed value.
This event is also emitted when a layer without visible history
has reached a new location. If you are only interested in changes that are visible in
the browser's address bar, observe up:location:changed instead.
The event is also emitted for location changes on the root layer.
This event is not emitted when an overlay is opened. For this, observe up:layer:opened instead.
