QIcon: add a hook in the engine so a non null QIconEngine can still be a null icon
QIcon: add a hook in the engine so a non null QIconEngine can still be a null icon

Implement it in the QIconLoader

We have to change detach() because some code does:
icon = QIcon::fromTheme("foobar"); if (icon.isNull()) icon.addPixmap(...);
so addPixmap and addFile have to work on a null QIcon by resetting
the iconEngine.

    Change-Id: I07719bef93930cf4692384a8c64e21a97dcce25c
    Reviewed-by: David Faure <david.faure@kdab.com>

libqtxdg/libqtxdg,lxde/libqtxdg,libqtxdg/libqtxdg,lxde/libqtxdg,luis-pereira/libqtxdg,luis-pereira/libqtxdg