Broken cast

All the way back since 2001 (ed53aaf5842e82f9a8d4d053b39e7db61c6d62ce "#89611#
react for (Beamer!)Frame->dispose() from outside," adding member
SfxChildWindow_Impl::xListener), that MyStruct hack was not in sync with
SfxChildWindow_Impl, so

  ((MyStruct*)pImp)->bVisible = false;

probably corrputed pFact rather than change bVisible.  The origin of that
MyStruct hack is lost to history, but probably stems from an ancient time when
changes across modules where prohibitively expensive, esp. when some feature had
to be crammed in shortly before release...

Change-Id: I13dc30c96ef70b2cc09dc117231c021910e0ce1e