connector widget status sync

[edublancas]

I found this scenarios where the connector widget doesn't properly sync the connection status:

• mark the existing connection as active: when connecting from the widget, the button shows as blue; however, if I connect using jupysql (%sql --section name), the button change to blue (note that there's an edge case: if I use %sql and connect to a database that's not defined in the ini file, the connector widget won't have any database record for it. in that case, it's fine
• Deleting a connection from the widget should also delete the connection from jupysql (i.e., running %sql --connections should no longer list the connection)
• if I connect to a non-default database, restarting the kernel won't restore the connection. to fix this, we can save some ~/.jupysql/state.json file and store the latest connected database identifier, so restarting the kernel, reconnects to it