Debugger: Better connection count display

Not a full fix yet. Disconnect does not reduce the count
since the Connection object remains in the structure,
only the receiver gets set to 0.

The connections are now listed as subitem of [signals]

Change-Id: I3dd795b3731b30e2af6f4d0e5da71dbde8996649
Reviewed-by: Eike Ziller <eike.ziller@digia.com>