PortalDialog: fix standard button handling

The top-level standardButtons property was aliased to something in a component, which then referred back to the top-level property! And the whole button box would only be visible if any of its buttons were shown.

This was fine in cases where implementations specified which buttons they wanted, but failed in the case where they wanted none and also specified their own set of custom actions.

Untangle this by specifying default buttons in the top-level property and making visibility take into account custom buttons.

As a cleanup, now we can remove all the requests for the standard buttons in the implementations (they'll fall back to the defaults).

I tested all implementations to make sure nothing regressed.

BUG: 519631
FIXED-IN: 6.6.5

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