network: Just dismiss forget dialog if response is not OK

If the dialog is deleted, for instance, we don't want to take that as
OK.

https://bugzilla.gnome.org/show_bug.cgi?id=709091
network: Get the correct widget holding the "connection" object data

The check button isn't a direct child of the GtkListBoxRow instance
that contains the "connection" object data so we are adding the wrong
widget to the "rows" list which then gives us a NULL for "connection"
in really_forget() and making us fail to remove the connection.

https://bugzilla.gnome.org/show_bug.cgi?id=709091
network: Destroy connection row widget when connection is removed

Otherwise all sorts of crashes may happen if it's interacted with.

https://bugzilla.gnome.org/show_bug.cgi?id=709104
network: Update forget button sensitivity when rows are destroyed

Doesn't make sense to keep it sensitive after the dialog returns OK.

This also avoids using finalized objects if the forget button is
clicked after some previously checked rows were destroyed because
their connection got removed elsewhere.

https://bugzilla.gnome.org/show_bug.cgi?id=709118