Ensure that the progress dialog parent is activated at the end under MSW.

The progress dialog parent was supposed to become the new foreground window
when the progress dialog was closed, but this didn't happen because
m_parentTop was never set when the native progress dialog implementation was
used under MSW. Fix this by explicitly calling the new SetTopParent() from its
ctor.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70512 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775