Listen to currentIndexChanged instead of activated.

This way we also get notified when the current item is
changed programatically. This then fixes the preselection
of the first item for the remote gammaray client.
Remember what the ConnectionFilterProxyModel is filtering on.

This fixes two things:

a) no more columnAdded/Removed signals when the receiver/sender
is set to null when the selection in the scene inspector is removed.
b) again, when the selection in the scene inspector is removed we
would show all connections even though we want to show none in such
a case.
Better debug output so we can figure out where the unimplemented model signals come from.
Fix typo and thus connection.