Revert "network: being able to use keyboard to connect to a Wireless"

This reverts commit d175a588f7949272e6a69bb956b6497a0b6897b0.

https://bugzilla.gnome.org/show_bug.cgi?id=710144
network: Don't use StButtons for items in selector

Their use blocks activation of the default button by keyboard, which
is important for accessibility. Use a Clutter.ClickAction instead,
which doesn't have this problem as it only considers mouse events.

https://bugzilla.gnome.org/show_bug.cgi?id=710144