Add active connection checking.

Add active connection checking.

QtSQL (pg driver) does not send any notification that the connection
to the database has terminated. LML tries to reconnect if a query
fails with a connection error. That check is not sufficient for the
objects that wait for DB notifications. This solution checks if the
connection is still alive by executing a simple query every minute.
If the query fails on a connection error the reconnect mechanism
triggers as before. A new reconnected() signal is emitted when a
reconnect occurs. The active connection checking is only done in
the main thread. Request-/Rename-handler listen for the new signal.