[connman-qt] Fix signal emission on property changes.

This fixes two issues. Setting the path on a network service
does not emit property changed signals. As a consequence that stored
state of the isConnected property may not relect its true value. Emits
the connected changed signal when the service goes offline for the
first time.
Merge pull request #110 from amccarthy/master

[connman-qt] Fix signal emission on property changes.