ActionListItem: Port away from BasicListItem (and some other stuff)

DiscoverWindow: Clean up currentTopLevel change handler

There is no third argument to PageRow::push() method, and API consumers are not required to provide second either if it's just an empty object. I couldn't even imagine what could that boolean result of status check mean, but I suppose it might be a thing of the past.

Also, window. qualifier isn't needed, since we are literally in top-level window's context.

ActionListItem, SearchField: Clean up property access, remove QML import versions

ActionListItem: Port away from BasicListItem

BasicListItem is deprecated. It is advised to use the new Kirigami Delegates module instead.

UpdateAction: bind visibility to enabled, just in case

By default ActionListItem binds visible: enabled, so this override should probably respect that, just in case updateAction may be disabled.

Though, probably we'd want some better diagnostics in such cases anyway, like a dummy page explaining that updates were disabled and why.