network: Avoid warnings while closing connection editor
It doesn't make sense to try to validate pages because widgets are
changing due to the whole dialog being destroyed and it causes a bunch
of warnings because some resources are cleared on each page dispose
method.
Avoid all that by disconnecting the page "changed" signal handler
before we start destroying widgets.
network: Don't try to get secrets for new connections
We don't need secrets for new connections and, in fact, trying to
retrieve secrets in that case will fail because we have a plain
NMConnection instead of a NMRemoteConnection.
The above mentioned error would result in the page never being
initialized.
