Support to read/save the plasmoid's state (Networking, Wireless, Wwan).

I had to use the same QTimer::singleShot hack I use with Wwan with the
other two to make sure NetworkManager will respect the plasmoid's
checkboxs status.

By the way, the Wireless checkbox is not working properly because of
a missing wirelesHardwareEnabledChanged emit in Solid's NM backend.
I already commited the fix to KDE/4.6 branch:
https://projects.kde.org/projects/kde/kdebase/kde-workspace/repository/revisions/2ec6533494a23164ee9c2de4ab65d5fa372aff5c