tweak browser.makeCurrentViewURL to preserve any query params that JBrowse does not know how to interpret
in dataset switching, rename current_dataset to dataset_id, remove assumptions about ?data= in URL from code and make them part of the configuration

references #134
only show the dataset selector if show_nav is true
decouple location-bar URL update from updating the 'Share' link.  fixes URL not being updated when in embedded mode