[libconnman-qt] don't allow defaultRoute that is not online state.

This aligns more closely with the convention of 'default route',
where there can be none.
[libconnman-qt] add serviceConnectionStarted and
serviceDisconnectionStarted signals.

These can be used by the ui to prepare for connection/disconnect.
[libconnman-qt] add connected property and signal.

This is a convience for 'online' and/or 'ready' status
Merge pull request #66 from lpotter/master

collection of convience add ons