Currently if a network connection's name is long enough, there will
be no space between the name label widget and the switch button widget
next to it.
Fix that by setting the end margin of label widget as 6 to keep some
space between those two widgets.
We need to block ourselves around the gtk_switch_set_active() call in
the error path, when we can't speak to the sharing dbus service, or we
end up in an endless loop since we're the ::state-set handler.
https://bugzilla.gnome.org/show_bug.cgi?id=739886
