Saving track configs in stateful URL.

Tracks in JBrowse view are saved/loaded in/from URL.

Behavior is enabled by default in JBrowse app. However, in order to turn it on in other apps (GenCover), JBrowse directive needs **keepState: true flag**. 

 
Then I'd need to fake item.id and item.static.name not to break up the execution. I think this is a more elegant way in the current solution, however some intermediate transformation code should be introduced (more in the comment below).