[Qt-creator] Designer plugin not loaded

[Olivier B.]

QtCreator used to load my custom widgets plugin. for this, i append to
QT_PLUGIN_PATH the path containing designer/libMywidgets.so
But this does not work anymore.
Looking at strace, i can see that the additional Qt_PLUGIN_PATH is
also scanned for other Qt plugin types, but not for designer plugins.
Even qtcreator's designer plugins folder is not scanned according to
the trace.
Help online also suggests going to look in Tools/Form Editor / About
designer plugins, but this menu entry does not exist
This is in 4.7.0, and 4.8.0 seems to do the same. Did designer plugins
usage change?