Change wxDIALOG_NO_PARENT to avoid clash with wxICON_EXCLAMATION.

Change wxDIALOG_NO_PARENT to avoid clash with wxICON_EXCLAMATION.

Use 0x20 (wxAPPLY) instead of 0x100 (wxICON_EXCLAMATION) for
wxDIALOG_NO_PARENT as otherwise using wxICON_EXCLAMATION with wxMessageBox in
wxGTK (and possibly other ports, although not wxMSW which doesn't honour
wxDIALOG_NO_PARENT for message boxes at all) resulted in not using the
specified parent for the message box and, as a side effect, not centering it
on its parent neither.

Closes #13464.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69112 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775