When the IP method is "disabled" or "shared" then everything else is
supposed to be insensitive. This currently fails if you toggle between
the two, because it's implemented using property bindings that are just
not smart enough to handle this task. Handle sensitivity only in
method_changed() to avoid this.
Additionally, not all of the widgets are being consistently
disabled/enabled when appropriate. E.g. when the method is "local" then
only the DNS entry, route entries, and default route checkboxes become
insensitive, leaving the other widgets, including notably the Automatic
switches, sensitive. They should all become insensitive, as when the
method is "disabled" or "shared." Fix this by organizing all the related
widgets into boxes and setting the sensitivity of the entire box. (Note
the strategy followed here does not exactly match nm-connection-editor,
which always allows editing addresses. We only allow that in Manual
mode. I'm not sure if this is advisable or not, so won't touch that.)
Finally, the Automatic DNS and Automatic Routes toggles should only be
sensitive when the method is "Automatic".
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