Improve jump-to-definition selector

[krassowski]

Needs to revive stale #41 and:

• show the source code for the jump target line, the line above and below (in addition to of the URI and line number) to inform the user about the context of available choices; this will require going to the JupyterLabWidgetAdapter to get the contents of the corresponding virtual files; if a file is not open, we will need to open it and get its content (but then the coordinates are absolute, not virtual!).
• strip the URI from the server root