Add EndDialog helper which calls EndModal with the given return code if the

Add EndDialog helper which calls EndModal with the given return code if the
dialog is modal, and Show(false) if the dialog is not modal.  This allows
the default handlers for OK and Cancel to function without assertions.
This code is copied more or less directly from src/msw/dialog.cpp.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@35730 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775