Improve main and extended messages handling in new wxMSW wxMessageDialog.

Improve main and extended messages handling in new wxMSW wxMessageDialog.

Don't use the main message if there is no extended message: this looks bad as
the main message is emphasized to contrast with the extended one which doesn't
make sense visually if there is no extended message.

Also recognize the common use of wxMessageBox() with a multiline text composed
of the first string that plays the role of the main message with the rest
being the extended one and handle this appropriately automatically. This
results in a better appearance by default for a lot of message boxes,
including even the one in our own minimal sample.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65466 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775