editor: fix secrets requests after 78b7101d
After 78b7101d, the page's connection would of course be the duplicated connection which is a plain NMConnection that user edits are done on. THat of course doesn't implement NMSettingsConnectionInterface, and thus can't respond to requests for secrets. Instead, we have the CEPage subclasses for each setting type pass back the name of the setting they want secrets for, if they need secrets, and let the editor ask for the secrets since it has the original connection which usually implements NMSettingsConnectionInterface. By moving the addition of the page's GtkWidget to after page initialization, we also fix system VPN connections, which were broken because the VPN page's plugin-provided widget is only valid after the page is initialized, but the editor expected it to be valid right after requesting secrets. In the case of system secrets, the request is over D-Bus and is async, which meant that the call would complete and the page would initalize long after the connection editor had tried to add the page to the UI.
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