[nemo-qml-plugin-connectivity] fix some functionality.

 Replacing QNetworkSession with connman-qt allows a single
 QNetworkRequest try.
 It also allows to get better error reporting and control if needed.

 Adding the dbus call to open the connectiondialog allows to get
 feedback to when user canceled the dialog.
Merge pull request #4 from lpotter/fix

 [nemo-qml-plugin-connectivity] fix some functionality.