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**. 

 
I am not sure if these extra `config.* ||` are needed.
The config extends these properties in [addTrack function](https://github.com/usoban/genapp-jbrowse/blob/feature-stateful-url/static/genapp-jbrowse/app/directives.js#L324)


I recently transformed the configuration: `config[genialisType].label` instead of `config.label`
If you add a label in [this configuration](https://github.com/usoban/genapp-jbrowse/blob/feature-stateful-url/static/genapp-jbrowse/app/controllers.js#L89), it will already overwrite the current one, without this line.