Prevent multiple JBrowse Desktops from pointing at the same config

[garrettjstevens]

Right now you can open two instances of JBrowse Desktop and point them at the same session, so they end up overwriting each other. One way to handle this would be to enforce that there is only a single JBrowse Desktop main thread running and each new session is a new window from that main thread. That would mean messages from the window to the main thread might have to contain info about which config it's viewing.

Electron provides app.requestSingleInstanceLock() to make sure only a single instance of an app is running.